# Dr. Andrei Manolescu

Position: Professor
Website: http://www.ru.is/starfsfolk/manoles

# Research projects

• Electronic states in core-shell nanowires
• Time-dependent transport at nanoscale

My research in physics is oriented to theoretical modelling and numerical calculations of quantum-mechanical electronic properties of semiconductor nanostructures. My work includes research on two-dimensional electron systems in magnetic fields, screening, exchange, and other many-body Coulomb phenomena, edge states, transport and electromagnetic absorption in modulated systems, electronic states in periodic electric and magnetic fields, spin polarization, magnetization. At present I am mostly involved in time dependent electronic transport in open nanosystems, charge polarization, and thermoelectric transport. I am interested in core-shell nanowires, in nanostructured solar cells, and in effects related to geometrical confinement of electrons at nanoscale.

# Publication

1. Vidar Gudmundsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
Self-induction and magnetic effects in electron transport through a photon cavity.
arXiv preprint arXiv:2005.10914 (2020).
Abstract We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the para- and diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field.
arXiv BibTeX

@article{gudmundsson2020self,
title = "Self-induction and magnetic effects in electron transport through a photon cavity",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Moldoveanu, Valeriu",
journal = "arXiv preprint arXiv:2005.10914",
year = 2020,
arxiv = "https://arxiv.org/abs/2005.10914",
abstract = "We explore higher order dynamical effects in the transport through a two-dimensional nanoscale electron system embedded in a three-dimensional far-infrared photon cavity. The nanoscale system is considered to be a short quantum wire with a single circular quantum dot defined in a GaAs heterostructure. The whole system, the external leads and the central system are placed in a constant perpendicular magnetic field. The Coulomb interaction of the electrons, the para- and diamagnetic electron-photon interactions are all treated by a numerically exact diagonalization using step-wise truncations of the appropriate many-body Fock spaces. We focus on the difference in transport properties between a description within an electric dipole approximation and a description including all higher order terms in a single photon mode model. We find small effects mostly caused by an electrical quadrupole and a magnetic dipole terms that depend strongly on the polarization of the cavity field with respect to the transport direction and the photon energy. When the polarization is aligned along the transport direction we find indications of a weak self-induction that we analyze and compare to the classical counterpart, and the self-energy contribution of high-order interaction terms to the states the electrons cascade through on their way through the system. Like expected the electron-photon interaction is well described in the dipole approximation when it is augmented by the lowest order diamagnetic part for a nanoscale system in a cavity in an external magnetic field."
}

2. George Alexandru Nemnes, Cristina Besleaga, Andrei Gabriel Tomulescu, Lucia Nicoleta Leonat, Viorica Stancu, Mihaela Florea, Andrei Manolescu and Ioana Pintilie.
The hysteresis-free behavior of perovskite solar cells from the perspective of the measurement conditions.
J. Mater. Chem. C 7, 5267-5274 (2019).
Abstract We investigate how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J–V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J–V scans, namely the bias scan rate and the bias poling voltage, and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J–V scans. Using HI, aging effects are also investigated, establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand, and the relaxation time scale of the slow process on the other hand. Pre-poling induced recombination effects are identified. In addition, our analysis based on sample pre-biasing reveals potential indications regarding two types of slow processes, with two different relaxation time scales, which provides further insight regarding ionic migration.
URL, DOI BibTeX

@article{C8TC05999C,
author = "Nemnes, George Alexandru and Besleaga, Cristina and Tomulescu, Andrei Gabriel and Leonat, Lucia Nicoleta and Stancu, Viorica and Florea, Mihaela and Manolescu, Andrei and Pintilie, Ioana",
title = "The hysteresis-free behavior of perovskite solar cells from the perspective of the measurement conditions",
journal = "J. Mater. Chem. C",
year = 2019,
volume = 7,
issue = 18,
pages = "5267-5274",
publisher = "The Royal Society of Chemistry",
doi = "10.1039/C8TC05999C",
url = "http://dx.doi.org/10.1039/C8TC05999C",
abstract = "We investigate how far the hysteresis-free behavior of perovskite solar cells can be reproduced using particular pre-conditioning and measurement conditions. As there are currently more and more reports of perovskite solar cells without J–V hysteresis it is crucial to distinguish between genuine performance improvements and measurement artifacts. We focus on two of the parameters that influence the dynamic J–V scans{,} namely the bias scan rate and the bias poling voltage{,} and point out measurement conditions for achieving a hysteresis-free behavior. In this context we discuss the suitability of defining a hysteresis index (HI) for the characterization of dynamic J–V scans. Using HI{,} aging effects are also investigated{,} establishing a potential connection between the sample degradation and the variation of the maximal hysteresis on one hand{,} and the relaxation time scale of the slow process on the other hand. Pre-poling induced recombination effects are identified. In addition{,} our analysis based on sample pre-biasing reveals potential indications regarding two types of slow processes{,} with two different relaxation time scales{,} which provides further insight regarding ionic migration."
}

3. GA Nemnes, TL Mitran and A Manolescu.
Gap prediction in hybrid graphene-hexagonal boron nitride nanoflakes using artificial neural networks.
Journal of Nanomaterials 2019 (2019).
Abstract The electronic properties of graphene nanoflakes (GNFs) with embedded hexagonal boron nitride (hBN) domains are investigated by combined ab initio density functional theory calculations and machine-learning techniques. The energy gaps of the quasi-0D graphene-based systems, defined as the differences between LUMO and HOMO energies, depend not only on the sizes of the hBN domains relative to the size of the pristine graphene nanoflake but also on the position of the hBN domain. The range of the energy gaps for different configurations increases as the hBN domains get larger. We develop two artificial neural network (ANN) models able to reproduce the gap energies with high accuracies and investigate the tunability of the energy gap, by considering a set of GNFs with embedded rectangular hBN domains. In one ANN model, the input is in one-to-one correspondence with the atoms in the GNF, while in the second model the inputs account for basic structures in the GNF, allowing potential use in upscaled systems. We perform a statistical analysis over different configurations of ANNs to optimize the network structure. The trained ANNs provide a correlation between the atomic system configuration and the magnitude of the energy gaps, which may be regarded as an efficient tool for optimizing the design of nanostructured graphene-based materials for specific electronic properties.
DOI BibTeX

@article{nemnes2019gap,
title = "Gap prediction in hybrid graphene-hexagonal boron nitride nanoflakes using artificial neural networks",
author = "Nemnes, GA and Mitran, TL and Manolescu, A",
journal = "Journal of Nanomaterials",
volume = 2019,
year = 2019,
publisher = "Hindawi",
doi = "10.1155/2019/6960787",
abstract = "The electronic properties of graphene nanoflakes (GNFs) with embedded hexagonal boron nitride (hBN) domains are investigated by combined ab initio density functional theory calculations and machine-learning techniques. The energy gaps of the quasi-0D graphene-based systems, defined as the differences between LUMO and HOMO energies, depend not only on the sizes of the hBN domains relative to the size of the pristine graphene nanoflake but also on the position of the hBN domain. The range of the energy gaps for different configurations increases as the hBN domains get larger. We develop two artificial neural network (ANN) models able to reproduce the gap energies with high accuracies and investigate the tunability of the energy gap, by considering a set of GNFs with embedded rectangular hBN domains. In one ANN model, the input is in one-to-one correspondence with the atoms in the GNF, while in the second model the inputs account for basic structures in the GNF, allowing potential use in upscaled systems. We perform a statistical analysis over different configurations of ANNs to optimize the network structure. The trained ANNs provide a correlation between the atomic system configuration and the magnitude of the energy gaps, which may be regarded as an efficient tool for optimizing the design of nanostructured graphene-based materials for specific electronic properties."
}

4. M Nita, M Tolea, A Manolescu and D C Marinescu.
HUND RULE OR MINIMAL SPIN IN CIRCULAR MOLECULES AND BIPARTITE LATTICES.
New Frontiers in Chemistry 28, 20-21 (2019).
Abstract Hund rule's relevance has long exceeded the boundaries of atomic physics where it was first formulated, over the years being investigated in many other systems such as quantum dots 1], artificial molecules created by quantum dots 2], metal clusters 3], bipartite lattices 4, 5], trilayer graphene 6], etc. In this paper we discuss the spin properties of circular molecules with an arbitrary number of atoms whose one-particle spectrum, in general, shows a ladder of double degenerate electronic states. We show 5] that an anti-Hund rule always defines the ground state in a circular molecule with 4N atoms at half-filling. In all other cases (i.e. number of atoms not multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction. In a celebrated paper, E. Lieb 7] gives exact results regarding the net spin of a socalled bipartite latice (i.e. which can be divided in "A" and "B" sub-groups of sites, hopping existing only between sites from different sub-groups) at half filling, in the presence of on-site Hubbard interaction. We study 8] the validity of the Lieb rule for a generalized Hamiltonian (including long-range interactions) using perturbatives plus Configuration-Interaction approaches.
URL BibTeX

@article{Nita2019hund,
author = "Nita,M. and Tolea,M. and Manolescu,A. and Marinescu,D. C.",
year = 2019,
title = "HUND RULE OR MINIMAL SPIN IN CIRCULAR MOLECULES AND BIPARTITE LATTICES",
journal = "New Frontiers in Chemistry",
volume = 28,
number = 1,
pages = "20-21",
note = "Copyright - Copyright West University of Timisoara, Faculty of Chemistry 2019; Last updated - 2020-04-02",
abstract = {Hund rule's relevance has long exceeded the boundaries of atomic physics where it was first formulated, over the years being investigated in many other systems such as quantum dots 1], artificial molecules created by quantum dots 2], metal clusters 3], bipartite lattices 4, 5], trilayer graphene 6], etc. In this paper we discuss the spin properties of circular molecules with an arbitrary number of atoms whose one-particle spectrum, in general, shows a ladder of double degenerate electronic states. We show 5] that an anti-Hund rule always defines the ground state in a circular molecule with 4N atoms at half-filling. In all other cases (i.e. number of atoms not multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction. In a celebrated paper, E. Lieb 7] gives exact results regarding the net spin of a socalled bipartite latice (i.e. which can be divided in "A" and "B" sub-groups of sites, hopping existing only between sites from different sub-groups) at half filling, in the presence of on-site Hubbard interaction. We study 8] the validity of the Lieb rule for a generalized Hamiltonian (including long-range interactions) using perturbatives plus Configuration-Interaction approaches.},
keywords = "Chemistry; Circularity; Atomic physics; Graphene; Quantum dots; Ground state; Onsite; Electron states; Atomic properties; Metal clusters",
isbn = 23932171,
language = "English",
url = "https://search.proquest.com/docview/2385400650?accountid=28419"
}

5. Vidar Gudmundsson, Hallmann Gestsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
Coexisting spin and Rabi oscillations at intermediate time regimes in electron transport through a photon cavity.
Beilstein Journal of Nanotechnology 10, 606-616 (2019).
DOI BibTeX

@article{Gudmundsson2019,
author = "Vidar Gudmundsson and Hallmann Gestsson and Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Valeriu Moldoveanu",
title = "Coexisting spin and Rabi oscillations at intermediate time regimes in electron transport through a photon cavity",
journal = "Beilstein Journal of Nanotechnology",
year = 2019,
volume = 10,
pages = "606-616",
issn = "2190-4286",
doi = "10.3762/bjnano.10.61",
}

6. Muhammad Taha Sultan, Andrei Manolescu, Jon Tomas Gudmundsson, Kristinn Torfason, George [Alexandru Nemnes], Ionel Stavarache, Constantin Logofatu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson.
Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing.
Applied Surface Science 469, 870 - 878 (2019).
Abstract Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified.
URL, DOI BibTeX

@article{SULTAN2019870,
title = "Enhanced photoconductivity of SiGe nanocrystals in SiO2 driven by mild annealing",
journal = "Applied Surface Science",
volume = 469,
pages = "870 - 878",
year = 2019,
issn = "0169-4332",
doi = "https://doi.org/10.1016/j.apsusc.2018.11.061",
url = "http://www.sciencedirect.com/science/article/pii/S0169433218331295",
author = "Muhammad Taha Sultan and Andrei Manolescu and Jon Tomas Gudmundsson and Kristinn Torfason and George [Alexandru Nemnes] and Ionel Stavarache and Constantin Logofatu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldor Gudfinnur Svavarsson",
keywords = "SiO, SiGe nanocrystals, Strain, Radio frequency magnetron sputtering, Thermal annealing, Photoconductivity",
abstract = "Photosensitive films based on finely dispersed semiconductor nanocrystals (NCs) in dielectric films have great potential for sensor applications. Here we report on preparation and characterization of photosensitive Si1-xGex NCs sandwiched between SiO2 matrix. A radio-frequency magnetron sputtering was applied to obtain a multilayer-structures (MLs) by depositing SiO2/SiGe/SiO2 films on Si (0 0 1) substrate. The Si1-xGex NCs were formed by a post-deposition annealing at 100–700 °C for 1–5 min. The effect of annealing temperature and time on MLs morphology and NCs size and density was studied using grazing incidence X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy and measurements of spectral distribution of photocurrent. It is demonstrated how the photoconductive properties of the MLs can be enhanced and tailored by controlling the NCs formation conditions and the presence of stress field in MLs and defects acting as traps and recombination centers. All these features can be adjusted/controlled by altering the annealing conditions (temperature and time). The MLs photosensitivity was increased of more than an order of magnitude by the annealing process. A mechanism, where a competition between crystallization process (NCs formation and evolution i.e. size and shapes) and stress field appearance determines the peak position in the photocurrent spectra, was identified."
}

7. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime.
arXiv preprint arXiv:1903.03655 (2019).
Abstract We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime.
arXiv BibTeX

@article{abdullah2019oscillations,
title = "Oscillations in electron transport caused by multiple resonances in a quantum dot-QED system in the steady-state regime",
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:1903.03655",
year = 2019,
arxiv = "https://arxiv.org/abs/1903.03655",
abstract = "We model the electron transport current as the photon energy is swept through several resonances of a multi-level quantum dot, embedded in a short quantum wire, coupled to photon cavity. We use a Markovian quantum master equation appropriate for the long-time evolution and include the electron-electron and both the para- and the diamagnetic electron-photon interactions via diagonalization in a truncated many-body Fock space. Tuning the photon energy, several anti-crossings caused by Rabi-splitting in the energy spectrum of the quantum dot system are found. The strength of the Rabi-splittings and the photon-exchange between the resonant states depend on the polarization of the cavity photon field. We observe oscillations of the charge in the system and several resonant transport current peaks for the photon energies corresponding to the resonances in the steady-state regime."
}

8. Maximilian M Sonner, Anna Sitek, Lisa Janker, Daniel Rudolph, Daniel Ruhstorfer, Markus Döblinger, Andrei Manolescu, Gerhard Abstreiter, Jonathan J Finley, Achim Wixforth, Gregor Koblmüller and Hubert J Krenner.
Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells.
Nano Letters 19, 3336-3343 (2019).
Abstract We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state.
URL, DOI BibTeX

@article{doi:10.1021/acs.nanolett.9b01028,
author = "Sonner, Maximilian M. and Sitek, Anna and Janker, Lisa and Rudolph, Daniel and Ruhstorfer, Daniel and Döblinger, Markus and Manolescu, Andrei and Abstreiter, Gerhard and Finley, Jonathan J. and Wixforth, Achim and Koblmüller, Gregor and Krenner, Hubert J.",
title = "Breakdown of Corner States and Carrier Localization by Monolayer Fluctuations in Radial Nanowire Quantum Wells",
journal = "Nano Letters",
volume = 19,
number = 5,
pages = "3336-3343",
year = 2019,
doi = "10.1021/acs.nanolett.9b01028",
note = "PMID: 31013103",
url = "https://doi.org/10.1021/acs.nanolett.9b01028",
eprint = "https://doi.org/10.1021/acs.nanolett.9b01028",
abstract = "We report a comprehensive study of the impact of the structural properties in radial GaAs-Al0.3Ga0.7As nanowire-quantum well heterostructures on the optical recombination dynamics and electrical transport properties, emphasizing particularly the role of the commonly observed variations of the quantum well thickness at different facets. Typical thickness fluctuations of the radial quantum well observed by transmission electron microscopy lead to pronounced localization. Our optical data exhibit clear spectral shifts and a multipeak structure of the emission for such asymmetric ring structures resulting from spatially separated, yet interconnected quantum well systems. Charge carrier dynamics induced by a surface acoustic wave are resolved and prove efficient carrier exchange on native, subnanosecond time scales within the heterostructure. Experimental findings are corroborated by theoretical modeling, which unambiguously show that electrons and holes localize on facets where the quantum well is the thickest and that even minute deviations of the perfect hexagonal shape strongly perturb the commonly assumed 6-fold symmetric ground state."
}

9. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Thermoelectric Inversion in a Resonant Quantum Dot-Cavity System in the Steady-State Regime.
Nanomaterials 9 (2019).
Abstract We theoretically investigate thermoelectric effects in a quantum dot system under the influence of a linearly polarized photon field confined to a 3D cavity. A temperature gradient is applied to the system via two electron reservoirs that are connected to each end of the quantum dot system. The thermoelectric current in the steady state is explored using a quantum master equation. In the presence of the quantized photons, extra channels, the photon replica states, are formed generating a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the energy difference between the lowest states of the quantum dot system is smaller than the photon energy, the thermoelectric current is almost blocked and a plateau is seen in the thermoelectric current for strong electron–photon coupling strength. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads.
URL, DOI BibTeX

@article{nano9050741,
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
title = "Thermoelectric Inversion in a Resonant Quantum Dot-Cavity System in the Steady-State Regime",
journal = "Nanomaterials",
volume = 9,
year = 2019,
number = 5,
article-number = 741,
url = "https://www.mdpi.com/2079-4991/9/5/741",
pubmedid = 31091757,
issn = "2079-4991",
abstract = "We theoretically investigate thermoelectric effects in a quantum dot system under the influence of a linearly polarized photon field confined to a 3D cavity. A temperature gradient is applied to the system via two electron reservoirs that are connected to each end of the quantum dot system. The thermoelectric current in the steady state is explored using a quantum master equation. In the presence of the quantized photons, extra channels, the photon replica states, are formed generating a photon-induced thermoelectric current. We observe that the photon replica states contribute to the transport irrespective of the direction of the thermal gradient. In the off-resonance regime, when the energy difference between the lowest states of the quantum dot system is smaller than the photon energy, the thermoelectric current is almost blocked and a plateau is seen in the thermoelectric current for strong electron–photon coupling strength. In the resonant regime, an inversion of thermoelectric current emerges due to the Rabi-splitting. Therefore, the photon field can change both the magnitude and the sign of the thermoelectric current induced by the temperature gradient in the absence of a voltage bias between the leads.",
doi = "10.3390/nano9050741"
}

10. G A Nemnes, T L Mitran, A Manolescu and Daniela Dragoman.
Electric and thermoelectric properties of graphene bilayers with extrinsic impurities under applied electric field.
Physica B: Condensed Matter 561, 9 - 15 (2019).
Abstract In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level.
URL, DOI BibTeX

@article{NEMNES20199,
title = "Electric and thermoelectric properties of graphene bilayers with extrinsic impurities under applied electric field",
journal = "Physica B: Condensed Matter",
volume = 561,
pages = "9 - 15",
year = 2019,
issn = "0921-4526",
doi = "https://doi.org/10.1016/j.physb.2019.02.044",
url = "http://www.sciencedirect.com/science/article/pii/S0921452619301334",
author = "G.A. Nemnes and T.L. Mitran and A. Manolescu and Daniela Dragoman",
keywords = "Bilayer graphene, Extrinsic doping, Field effect, Seebeck coefficient",
abstract = "In contrast to monolayer graphene, in bilayer graphene (BLG) one can induce a tunable bandgap by applying an external electric field, which makes it suitable for field effect applications. Extrinsic doping of BLGs enriches the electronic properties of the graphene-based family, as their behavior can be switched from an intrinsic small-gap semiconductor to a degenerate semiconductor. In the framework of density functional theory (DFT) calculations, we investigate the electronic and thermoelectric properties of BLGs doped with extrinsic impurities from groups III (B, Al, Ga), IV (Si, Ge) and V (N, P, As), in the context of applied external electric fields. Doping one monolayer of the BLG with p- or n-type dopants results in a degenerate semiconductor, where the Fermi energy depends on the type of the impurity, but also on the magnitude and orientation of the electric field, which modifies the effective doping concentration. Doping one layer with isoelectronic species like Si and Ge opens a gap, which may be closed upon applying an electric field, in contrast to the pristine BLG. Furthermore, dual doping by III-V elements, in a way that the BLG system is formed by one n-type and one p-type graphene monolayer, leads to intrinsic semiconductor properties with relatively large energy gaps. Si-Si and Ge-Ge substitutions render a metallic like behavior at zero field similar to the standard BLG, however with an asymmetric density of states in the vicinity of the Fermi energy. We analyze the suitability of the highly doped BLG materials for thermoelectric applications, exploiting the large asymmetries of the density of states. In addition, a sign change in the Seebeck coefficient is observed by tuning the electric field as a signature of narrow bands near the Fermi level."
}

11. M T Sultan, J T Gudmundsson, A Manolescu, T Stoica, M L Ciurea and H G Svavarsson.
Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation.
Applied Surface Science 479, 403 - 409 (2019).
Abstract We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent.
URL, DOI BibTeX

@article{SULTAN2019403,
title = "Enhanced photoconductivity of embedded SiGe nanoparticles by hydrogenation",
journal = "Applied Surface Science",
volume = 479,
pages = "403 - 409",
year = 2019,
issn = "0169-4332",
doi = "https://doi.org/10.1016/j.apsusc.2019.02.096",
url = "http://www.sciencedirect.com/science/article/pii/S0169433219304398",
author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and T. Stoica and M.L. Ciurea and H.G. Svavarsson",
keywords = "SiGe, SiO, Nanoparticles, Photocurrent, Hydrogen plasma, Passivation",
abstract = "We investigate the effect of room-temperature hydrogen-plasma treatment on the photoconductivity of SiGe nanoparticles sandwiched within SiO2 layers. An increase in photocurrent intensity of more than an order magnitude is observed after the hydrogen plasma treatment. The enhancement is attributed to neutralization of dangling bonds at the nanoparticles and to passivation of nonradiative defects in the oxide matrix and at SiGe/matrix interfaces. We find that increasing the partial pressure of hydrogen to pressures where H3+ and H2+ were the dominant ions results in increased photocurrent."
}

12. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
Efficacy of annealing and fabrication parameters on photo-response of SiGe in TiO2 matrix.
Nanotechnology 30, 365604 (June 2019).
Abstract SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures.
URL, DOI BibTeX

@article{Sultan_2019,
doi = "10.1088/1361-6528/ab260e",
url = "https://doi.org/10.1088%2F1361-6528%2Fab260e",
year = 2019,
month = "jun",
publisher = "{IOP} Publishing",
volume = 30,
number = 36,
pages = 365604,
author = "M T Sultan and J T Gudmundsson and A Manolescu and V S Teodorescu and M L Ciurea and H G Svavarsson",
title = "Efficacy of annealing and fabrication parameters on photo-response of {SiGe} in {TiO}2 matrix",
journal = "Nanotechnology",
abstract = "SiGe nanoparticles dispersed in a dielectric matrix exhibit properties different from those of bulk and have shown great potential in devices for application in advanced optoelectronics. Annealing is a common fabrication step used to increase crystallinity and to form nanoparticles in such a system. A frequent downside of such annealing treatment is the formation of insulating SiO2 layer at the matrix/SiGe interface, degrading the optical properties of the structure. An annealing process that could bypass this downside would therefore be of great interest. In this work, a short-time furnace annealing of a SiGe/TiO2 system is applied to obtain SiGe nanoparticles without formation of the undesired SiO2 layer between the dielectric matrix (TiO2) and SiGe. The structures were prepared by depositing alternate layers of TiO2 and SiGe films, using direct-current magnetron sputtering technique. A wide range spectral response with a response-threshold up to ∼1300 nm was obtained, accompanied with an increase in photo-response of more than two-orders of magnitude. Scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy and grazing incidence x-ray diffraction were used to analyze the morphological changes in respective structures. Photoconductive properties were studied by measuring photocurrent spectra using applied dc-voltages at various temperatures."
}

13. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Manifestation of the Purcell Effect in Current Transport through a Dot–Cavity–QED System.
Nanomaterials 9 (2019).
Abstract We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity–photon reservoir coupling, the mean photon number in the reservoir, the electron–photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron–photon coupling strength. In the selected range of the parameters, the electron–photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity–reservoir coupling, while they decrease with increasing electron–photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.
URL, DOI BibTeX

@article{nano9071023,
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
title = "Manifestation of the Purcell Effect in Current Transport through a Dot–Cavity–QED System",
journal = "Nanomaterials",
volume = 9,
year = 2019,
number = 7,
article-number = 1023,
url = "https://www.mdpi.com/2079-4991/9/7/1023",
pubmedid = 31319544,
issn = "2079-4991",
abstract = "We study the transport properties of a wire-dot system coupled to a cavity and a photon reservoir. The system is considered to be microstructured from a two-dimensional electron gas in a GaAs heterostructure. The 3D photon cavity is active in the far-infrared or the terahertz regime. Tuning the photon energy, Rabi-resonant states emerge and in turn resonant current peaks are observed. We demonstrate the effects of the cavity–photon reservoir coupling, the mean photon number in the reservoir, the electron–photon coupling and the photon polarization on the intraband transitions occurring between the Rabi-resonant states, and on the corresponding resonant current peaks. The Rabi-splitting can be controlled by the photon polarization and the electron–photon coupling strength. In the selected range of the parameters, the electron–photon coupling and the cavity-environment coupling strengths, we observe the results of the Purcell effect enhancing the current peaks through the cavity by increasing the cavity–reservoir coupling, while they decrease with increasing electron–photon coupling. In addition, the resonant current peaks are also sensitive to the mean number of photons in the reservoir.",
doi = "10.3390/nano9071023"
}

14. M U Torres, A Sitek and A Manolescu.
Transverse polarization light scattering in tubular semiconductor nanowires.
In 2019 21st International Conference on Transparent Optical Networks (ICTON) (). (2019), 1-4.
BibTeX

@inproceedings{8840569,
author = "M. U. {Torres} and A. {Sitek} and A. {Manolescu}",
booktitle = "2019 21st International Conference on Transparent Optical Networks (ICTON)",
title = "Transverse polarization light scattering in tubular semiconductor nanowires",
year = 2019,
volume = "",
number = "",
pages = "1-4"
}

15. Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Generalized Master Equation Approach to Time-Dependent Many-Body Transport.
Entropy 21 (2019).
Abstract We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron–photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima–Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov–Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A many-body Markovian version of the GME for cavity-coupled systems is also presented.
URL, DOI BibTeX

@article{e21080731,
author = "Moldoveanu, Valeriu and Manolescu, Andrei and Gudmundsson, Vidar",
title = "Generalized Master Equation Approach to Time-Dependent Many-Body Transport",
journal = "Entropy",
volume = 21,
year = 2019,
number = 8,
article-number = 731,
url = "https://www.mdpi.com/1099-4300/21/8/731",
issn = "1099-4300",
abstract = "We recall theoretical studies on transient transport through interacting mesoscopic systems. It is shown that a generalized master equation (GME) written and solved in terms of many-body states provides the suitable formal framework to capture both the effects of the Coulomb interaction and electron–photon coupling due to a surrounding single-mode cavity. We outline the derivation of this equation within the Nakajima–Zwanzig formalism and point out technical problems related to its numerical implementation for more realistic systems which can neither be described by non-interacting two-level models nor by a steady-state Markov–Lindblad equation. We first solve the GME for a lattice model and discuss the dynamics of many-body states in a two-dimensional nanowire, the dynamical onset of the current-current correlations in electrostatically coupled parallel quantum dots and transient thermoelectric properties. Secondly, we rely on a continuous model to get the Rabi oscillations of the photocurrent through a double-dot etched in a nanowire and embedded in a quantum cavity. A many-body Markovian version of the GME for cavity-coupled systems is also presented.",
doi = "10.3390/e21080731"
}

16. Anna Sitek, Miguel Urbaneja Torres and Andrei Manolescu.
Corner and side localization of electrons in irregular hexagonal semiconductor shells.
Nanotechnology 30, 454001 (August 2019).
Abstract We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core–shell nanowires built of III–V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core–shell nanowires.
URL, DOI BibTeX

@article{Sitek_2019,
doi = "10.1088/1361-6528/ab37a1",
url = "https://doi.org/10.1088%2F1361-6528%2Fab37a1",
year = 2019,
month = "aug",
publisher = "{IOP} Publishing",
volume = 30,
number = 45,
pages = 454001,
author = "Anna Sitek and Miguel Urbaneja Torres and Andrei Manolescu",
title = "Corner and side localization of electrons in irregular hexagonal semiconductor shells",
journal = "Nanotechnology",
abstract = "We discuss the low energy electronic states in hexagonal rings. These states correspond to the transverse modes in core–shell nanowires built of III–V semiconductors which typically have a hexagonal cross section. In the case of symmetric structures the 12 lowest states (including the spin) are localized in the corners, while the next following 12 states are localized mostly on the sides. Depending on the material parameters, in particular the effective mass, the ring diameter and width, the corner and side states may be separated by a considerable energy gap, ranging from few to tens of meV. In a realistic fabrication process geometric asymmetries are unavoidable, and therefore the particles are not symmetrically distributed between all corner and side areas. Possibly, even small deformations may shift the localization of the ground state to one of the sides. The transverse states or the transitions between them may be important in transport or optical experiments. Still, up to date, there are only very few experimental investigations of the localization-dependent properties of core–shell nanowires."
}

17. Miguel Urbaneja Torres, Anna Sitek and Andrei Manolescu.
Anisotropic light scattering by prismatic semiconductor nanowires.
Opt. Express 27, 25502–25514 (September 2019).
Abstract Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing.
URL, DOI BibTeX

@article{UrbanejaTorres:19,
author = "Miguel Urbaneja Torres and Anna Sitek and Andrei Manolescu",
journal = "Opt. Express",
keywords = "Forward scattering; Guided mode resonance; Light matter interactions; Light scattering; Mie resonances; Resonant modes",
number = 18,
pages = "25502--25514",
publisher = "OSA",
title = "Anisotropic light scattering by prismatic semiconductor nanowires",
volume = 27,
month = "Sep",
year = 2019,
url = "http://www.opticsexpress.org/abstract.cfm?URI=oe-27-18-25502",
doi = "10.1364/OE.27.025502",
abstract = "Anisotropic transverse light scattering by prismatic nanowires is a natural outcome of their geometry. In this work, we perform numerical calculations of the light scattering characteristics for nanowires in the optical and near-infrared range and explore the possibility of tuning the directivity by changing the angle of light incidence. The scattering cross section and the directivity of the scattered light when it is incident perpendicular to a facet or to an edge of the prism are investigated both with transverse electric and with transverse magnetic polarizations. The phenomenology includes Mie resonances and guided modes yielding together rich and complex spectra. We consider nanowires with hexagonal, square and triangular cross sections. The modes that are most sensitive to the incidence angle are the hexapole for the hexagonal case and the quadrupole for the square case. Higher order modes are also sensitive, but mostly for the square geometry. Our results indicate the possibility of a flexible in-situ tunability of the directivity simply by rotating the nanowire profile relatively to the direction of the incident light which could offer potential advantages in applications such as switching or sensing."
}

18. Valeriu Moldoveanu, Ion Viorel Dinu, Andrei Manolescu and Vidar Gudmundsson.
Backaction effects in cavity-coupled quantum conductors.
Phys. Rev. B 100, 125416 (September 2019).
Abstract We study the electronic transport through a pair of distant nanosystems (Sa and Sb) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem Sb induces a photon-assisted current in the other subsystem Sa which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of backaction, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system.
URL, DOI BibTeX

@article{PhysRevB.100.125416,
title = "Backaction effects in cavity-coupled quantum conductors",
author = "Moldoveanu, Valeriu and Dinu, Ion Viorel and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 100,
issue = 12,
pages = 125416,
numpages = 9,
year = 2019,
month = "Sep",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.100.125416",
abstract = "We study the electronic transport through a pair of distant nanosystems (Sa and Sb) embedded in a single-mode cavity. Each system is connected to source and drain particle reservoirs and the electron-photon coupling is described by the Tavis-Cummings model. The generalized master equation approach provides the reduced density operator of the double system in the dressed-states basis. It is shown that the photon-mediated coupling between the two subsystems leaves a signature on their transient and steady-state currents. In particular, a suitable bias applied on subsystem Sb induces a photon-assisted current in the other subsystem Sa which is otherwise in the Coulomb blockade. We also predict that a transient current passing through one subsystem triggers a charge transfer between the optically active levels of the second subsystem even if the latter is not connected to the leads. As a result of backaction, the transient current through the open system develops Rabi oscillations (ROs) whose period depends on the initial state of the closed system."
}

19. Muhammad Taha Sultan, Adrian Valentin Maraloiu, Ionel Stavarache, Tómas Jón Gudmundsson, Andrei Manolescu, Valentin Serban Teodorescu, Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson.
Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study.
Beilstein Journal of Nanotechnology 10, 1873-1882 (2019).
Abstract Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra.
DOI BibTeX

@article{Sultan2019,
author = "Muhammad Taha Sultan and Adrian Valentin Maraloiu and Ionel Stavarache and Jón Tómas Gudmundsson and Andrei Manolescu and Valentin Serban Teodorescu and Magdalena Lidia Ciurea and Halldór Gudfinnur Svavarsson",
title = "Fabrication and characterization of Si1−xGex nanocrystals in as-grown and annealed structures: a comparative study",
journal = "Beilstein Journal of Nanotechnology",
year = 2019,
volume = 10,
pages = "1873-1882",
issn = "2190-4286",
doi = "10.3762/bjnano.10.182",
abstract = "Multilayer structures comprising of SiO2/SiGe/SiO2 and containing SiGe nanoparticles were obtained by depositing SiO2 layers using reactive direct current magnetron sputtering (dcMS), whereas, Si and Ge were co-sputtered using dcMS and high-power impulse magnetron sputtering (HiPIMS). The as-grown structures subsequently underwent rapid thermal annealing (550–900 °C for 1 min) in N2 ambient atmosphere. The structures were investigated using X-ray diffraction, high-resolution transmission electron microscopy together with spectral photocurrent measurements, to explore structural changes and corresponding properties. It is observed that the employment of HiPIMS facilitates the formation of SiGe nanoparticles (2.1 ± 0.8 nm) in the as-grown structure, and that presence of such nanoparticles acts as a seed for heterogeneous nucleation, which upon annealing results in the periodically arranged columnar self-assembly of SiGe core–shell nanocrystals. An increase in photocurrent intensity by more than an order of magnitude was achieved by annealing. Furthermore, a detailed discussion is provided on strain development within the structures, the consequential interface characteristics and its effect on the photocurrent spectra."
}

20. D V Anghel, G A Nemnes, Ioana Pintilie and A Manolescu.
Modelling J\textendashV hysteresis in perovskite solar cells induced by voltage poling.
Physica Scripta 94, 125809 (October 2019).
Abstract We present an extension of the dynamic electrical model, which enable us to explain some important features of the perovskite solar cells (PSC), like the shape of the hysteresis and the appearance of the ’bump’ in the so called reverse scan, without requiring any additional assumptions. We give analytical expressions in terms of the Lambert’s function W for the open circuit voltage, the stationary current, and the instantaneous current, which can be written also in terms of elementary functions for the most part of the ranges of the physical parameters. The initial polarization of the cell, modeled as the charging of a capacitor with voltage dependent capacitance, is consistently determined in the model, from the initial stationary conditions. This is inline with a previously observed sharp increase of the PSC capacitance beyond the open-circuit voltage. Besides the known features, we obtain characteristics that were not yet analyzed experimentally, like the change of the bump from the reverse scan branch of the J–V characteristic to the forward scan, with the increase of the poling voltage (or the increase of the PSC capacitance).
URL, DOI BibTeX

@article{Anghel_2019,
doi = "10.1088/1402-4896/ab347d",
url = "https://doi.org/10.1088%2F1402-4896%2Fab347d",
year = 2019,
month = "oct",
publisher = "{IOP} Publishing",
volume = 94,
number = 12,
pages = 125809,
author = "D V Anghel and G A Nemnes and Ioana Pintilie and A Manolescu",
title = "Modelling J{\textendash}V hysteresis in perovskite solar cells induced by voltage poling",
journal = "Physica Scripta",
abstract = "We present an extension of the dynamic electrical model, which enable us to explain some important features of the perovskite solar cells (PSC), like the shape of the hysteresis and the appearance of the ’bump’ in the so called reverse scan, without requiring any additional assumptions. We give analytical expressions in terms of the Lambert’s function W for the open circuit voltage, the stationary current, and the instantaneous current, which can be written also in terms of elementary functions for the most part of the ranges of the physical parameters. The initial polarization of the cell, modeled as the charging of a capacitor with voltage dependent capacitance, is consistently determined in the model, from the initial stationary conditions. This is inline with a previously observed sharp increase of the PSC capacitance beyond the open-circuit voltage. Besides the known features, we obtain characteristics that were not yet analyzed experimentally, like the change of the bump from the reverse scan branch of the J–V characteristic to the forward scan, with the increase of the poling voltage (or the increase of the PSC capacitance)."
}

21. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
The photocurrent generated by photon replica states of an off-resonantly coupled dot-cavity system.
Scientific reports 9, 1–10 (2019).
Abstract Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons. Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive to the photon polarization, but it is not strongly dependent on the average number of photons in the environment.
DOI BibTeX

@article{abdullah2019photocurrent,
title = "The photocurrent generated by photon replica states of an off-resonantly coupled dot-cavity system",
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Scientific reports",
volume = 9,
number = 1,
pages = "1--10",
year = 2019,
publisher = "Nature Publishing Group",
doi = "10.1038/s41598-019-51320-8",
abstract = "Transport properties of a quantum dot coupled to a photon cavity are investigated using a quantum master equation in the steady-state regime. In the off-resonance regime, when the photon energy is smaller than the energy spacing between the lowest electron states of the quantum dot, we calculate the current that is generated by photon replica states as the electronic system is pumped with photons. Tuning the electron-photon coupling strength, the photocurrent can be enhanced by the influences of the photon polarization, and the cavity-photon coupling strength of the environment. We show that the current generated through the photon replicas is very sensitive to the photon polarization, but it is not strongly dependent on the average number of photons in the environment."
}

22. Vidar Gudmundsson, Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Valeriu Moldoveanu.
Cavity-Photon-Induced High-Order Transitions between Ground States of Quantum Dots.
Annalen der Physik 531, 1900306 (2019).
Abstract Abstract It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para- and diamagnetic electron–photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.
URL, DOI BibTeX

@article{doi:10.1002/andp.201900306,
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Moldoveanu, Valeriu",
title = "Cavity-Photon-Induced High-Order Transitions between Ground States of Quantum Dots",
journal = "Annalen der Physik",
volume = 531,
number = 11,
pages = 1900306,
doi = "10.1002/andp.201900306",
url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.201900306",
eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201900306",
abstract = "Abstract It is shown that quantum electromagnetic transitions to high orders are essential to describe the time-dependent path of a nanoscale electron system in a Coulomb blockade regime when coupled to external leads and placed in a 3D rectangular photon cavity. The electronic system consists of two quantum dots embedded asymmetrically in a short quantum wire. The two lowest in energy spin degenerate electron states are mostly localized in each dot with only a tiny probability in the other dot. In the presence of the leads, a slow high-order transition between the ground states of the two quantum dots is identified. The Fourier power spectrum for photon–photon correlations in the steady state shows a Fano type of resonance for the frequency of the slow transition. Full account is taken of the geometry of the multilevel electronic system, and the electron–electron Coulomb interactions together with the para- and diamagnetic electron–photon interactions are treated with step-wise exact numerical diagonalization and truncation of appropriate many-body Fock spaces. The matrix elements for all interactions are computed analytically or numerically exactly.",
year = 2019
}

23. Miguel Urbaneja Torres, Kristjan Ottar Klausen, Anna Sitek, Sigurdur I Erlingsson, Vidar Gudmundsson and Andrei Manolescu.
Anisotropic electromagnetic field emitted by core-shell semiconductor nanowires driven by an alternating current.
arXiv e-prints, pages arXiv:1912.10284 (December 2019).
Abstract We consider tubular nanowires with polygonal cross-section, whose geometry has important implications for the electron localization and separates the lowest energy groups into two sets of corner and side-localized states. The presence of an external magnetic field transversal to the nanowires imposes an additional localization mechanism, the electrons being pushed to the sides relative to the direction of the field. This has important implications on the current density, as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that, because of the aforementioned localization properties, the radiated field has an anisotropic behavior that can reproduce the internal geometry of the nanowire. Consequently, core-shell nanowires made of semiconductor materials, driven by an alternating current, may act as an anisotropic emitter nanoantenna, at least in the neighborhood of the nanowire.
arXiv BibTeX

@article{2019arXiv191210284U,
author = "{Urbaneja Torres}, Miguel and {Klausen}, Kristjan Ottar and {Sitek}, Anna and {Erlingsson}, Sigurdur I. and {Gudmundsson}, Vidar and {Manolescu}, Andrei",
title = "{Anisotropic electromagnetic field emitted by core-shell semiconductor nanowires driven by an alternating current}",
journal = "arXiv e-prints",
keywords = "Condensed Matter - Mesoscale and Nanoscale Physics",
year = 2019,
month = 12,
eid = "arXiv:1912.10284",
pages = "arXiv:1912.10284",
archiveprefix = "arXiv",
eprint = "1912.10284",
primaryclass = "cond-mat.mes-hall",
adsnote = "Provided by the SAO/NASA Astrophysics Data System",
arxiv = "https://arxiv.org/abs/1912.10284",
abstract = "We consider tubular nanowires with polygonal cross-section, whose geometry has important implications for the electron localization and separates the lowest energy groups into two sets of corner and side-localized states. The presence of an external magnetic field transversal to the nanowires imposes an additional localization mechanism, the electrons being pushed to the sides relative to the direction of the field. This has important implications on the current density, as it creates current loops induced by the Lorentz force. We calculate numerically the electromagnetic field radiated by hexagonal, square, and triangular nanowires. We demonstrate that, because of the aforementioned localization properties, the radiated field has an anisotropic behavior that can reproduce the internal geometry of the nanowire. Consequently, core-shell nanowires made of semiconductor materials, driven by an alternating current, may act as an anisotropic emitter nanoantenna, at least in the neighborhood of the nanowire."
}

24. Kristján Óttar Klausen, Anna Sitek, Sigurdur Ingi Erlingsson and Andrei Manolescu.
Majorana Zero Modes in Nanowires with Combined Triangular and Hexagonal Geometry.
Nanotechnology (2020).
Abstract The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with the same core and shell geometry. We suggest that a dual core nanowire is needed to allow for braiding operation of Majorana zero modes at the nanowire end plane.
URL, DOI BibTeX

@article{10.1088/1361-6528/ab932e,
author = "Kristján Óttar Klausen and Anna Sitek and Sigurdur Ingi Erlingsson and Andrei Manolescu",
title = "Majorana Zero Modes in Nanowires with Combined Triangular and Hexagonal Geometry",
journal = "Nanotechnology",
url = "http://iopscience.iop.org/10.1088/1361-6528/ab932e",
year = 2020,
doi = "10.1088/1361-6528",
abstract = "The effects of geometry on the hosting of Majorana zero modes are explored in core-shell nanowires with a hexagonal core and a triangular shell, and vice versa. The energy interval separating electronic states localized in the corners from states localized on the sides of the shell is shown to be larger for a triangular nanowire with a hexagonal core, than a triangular one. We build the topological phase diagram for both cases and compare them to earlier work on prismatic nanowires with the same core and shell geometry. We suggest that a dual core nanowire is needed to allow for braiding operation of Majorana zero modes at the nanowire end plane."
}

25. M T Sultan, J T Gudmundsson, A Manolescu, V S Teodorescu, M L Ciurea and H G Svavarsson.
Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing.
Applied Surface Science 511, 145552 (2020).
Abstract Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering.
URL, DOI BibTeX

@article{SULTAN2020145552,
title = "Obtaining SiGe nanocrystallites between crystalline TiO2 layers by HiPIMS without annealing",
journal = "Applied Surface Science",
volume = 511,
pages = 145552,
year = 2020,
issn = "0169-4332",
doi = "10.1016/j.apsusc.2020.145552",
url = "http://www.sciencedirect.com/science/article/pii/S0169433220303081",
author = "M.T. Sultan and J.T. Gudmundsson and A. Manolescu and V.S. Teodorescu and M.L. Ciurea and H.G. Svavarsson",
keywords = "TiO, SiGe, Nanoparticles, HiPIMS, GiXRD, Interface, Photo-spectra",
abstract = "Formation of SiGe nanocrystals in an oxide matrix via deposition and subsequent annealing is a widely applied approach as it gives good control over optical properties by varying the Ge atomic fraction, the size, shape and crystallinity of the nanocrystals. A common drawback of annealing is a strain relaxation in the structure creating dislocations, point defects, dangling bonds, Ge clustering and altered interface morphology. All these phenomena are well-known to degrade the optoelectronic and electrical properties of the structure. As a proof of concept, in this study we have utilized a modern technique of high impulse power magnetron sputtering (HiPIMS) to obtain a crystalline TiO2/SiGe/TiO2 structure without any pre-/post-annealing. It is furthermore demonstrated how a control of the nano-crystallite size is obtained by altering the HiPIMS discharge power alone. Grazing incidence X-ray diffraction analysis was carried out for the structural characterization, while photocurrent measurements were utilized to access the role of TiO2 structural morphology over interface integrity in determining spectral feature and sensitivity. An increase of 1 – 2 orders magnitude in spectral intensity was achieved for as-grown structures fabricated via HiPIMS in comparison to annealed structure, sputtered with conventional direct current magnetron sputtering."
}

26. Nzar Rauf Abdullah, Hunar Omar Rashid, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Modeling electronic, mechanical, optical and thermal properties of graphene-like BC $_6$ N materials: Role of prominent BN-bonds.
arXiv preprint arXiv:2003.08467 (2020).
Abstract We model monolayer graphene-like materials with BC6N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC6N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC6N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC6N without BN-bonds has more dynamical stability. Furthermore, all the BC6N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC6N systems caused by hot carrier–assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices.
arXiv BibTeX

@article{abdullah2020modeling,
title = "Modeling electronic, mechanical, optical and thermal properties of graphene-like BC $\_6$ N materials: Role of prominent BN-bonds",
author = "Abdullah, Nzar Rauf and Rashid, Hunar Omar and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:2003.08467",
year = 2020,
arxiv = "https://arxiv.org/abs/2003.08467",
abstract = "We model monolayer graphene-like materials with BC6N stoichiometry where the bonding between the B and the N atoms plays an important role for their physical and chemical properties. Two types of BC6N are found based on the BN bonds: In the presence of BN bonds, an even number of π-bonds emerges indicating an aromatic structure and a large direct bandgap appears, while in the absence of BN bonds, an anti-aromatic structure with an odd-number of π-bonds is found resulting a direct small bandgap. The stress-strain curves shows high elastic moduli and tensile strength of the structures with BN-bonds, compared to structures without BN-bonds. Self-consistent field calculations demonstrate that BC6N with BN-bonds is energetically more stable than structures without BN-bonds due to a strong binding energy between the B and the N atoms, while their phonon dispersion displays that BC6N without BN-bonds has more dynamical stability. Furthermore, all the BC6N structures considered show a large absorption of electromagnetic radiation with polarization parallel to the monolayers in the visible range. Finer detail of the absorption depend on the actual structures of the layers. A higher electronic thermal conductivity and specific heat are seen in BC6N systems caused by hot carrier--assisted charge transport. This opens up a possible optimization for bolometric applications of graphene based material devices."
}

27. Nzar Rauf Abdullah, Hunar Omar Rashid, Mohammad T Kareem, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Effects of bonded and non-bonded B/N codoping of graphene on its stability, interaction energy, electronic structure, and power factor.
Physics Letters A 384, 126350 (2020).
Abstract We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms.
URL, DOI BibTeX

@article{ABDULLAH2020126350,
title = "Effects of bonded and non-bonded B/N codoping of graphene on its stability, interaction energy, electronic structure, and power factor",
journal = "Physics Letters A",
volume = 384,
number = 12,
pages = 126350,
year = 2020,
issn = "0375-9601",
doi = "https://doi.org/10.1016/j.physleta.2020.126350",
url = "http://www.sciencedirect.com/science/article/pii/S0375960120301602",
author = "Nzar Rauf Abdullah and Hunar Omar Rashid and Mohammad T. Kareem and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
keywords = "Energy harvesting, Thermal transport, Graphene, Density functional theory, Electronic structure",
abstract = "We model boron and nitrogen doped/codoped monolayer graphene to study its stability, interaction energy, electronic and thermal properties using density functional theory. It is found that a doped graphene sheet with non-bonded B or N atoms induces an attractive interaction and thus opens up the bandgap. Consequently, the power factor is enhanced. Additionally, bonded B or N atoms in doped graphene generate a repulsive interaction leading to a diminished bandgap, and thus a decreased power factor. We emphasis that enhancement of the power factor is not very sensitive to the concentration of the boron and nitrogen atoms, but it is more sensitive to the positions of the B or N atoms in ortho, meta, and para positions of the hexagonal structure of graphene. In the B and N codoped graphene, the non-bonded dopant atoms have a weak attractive interaction and interaction leading to a small bandgap, while bonded doping atoms cause a strong attractive interaction and a large bandgap. As a result, the power factor of the graphene with non-bonded doping atoms is reduced while it is enhanced for graphene with bonded doping atoms."
}

28. Ionel Stavarache, Constantin Logofatu, Muhammad Taha Sultan, Andrei Manolescu, Halldor Gudfinnur Svavarsson, Valentin Serban Teodorescu and Magdalena Lidia Ciurea.
SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared.
Scientific Reports 10, 1–9 (2020).
Abstract Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.
DOI BibTeX

@article{stavarache2020sige,
title = "SiGe nanocrystals in SiO 2 with high photosensitivity from visible to short-wave infrared",
author = "Stavarache, Ionel and Logofatu, Constantin and Sultan, Muhammad Taha and Manolescu, Andrei and Svavarsson, Halldor Gudfinnur and Teodorescu, Valentin Serban and Ciurea, Magdalena Lidia",
journal = "Scientific Reports",
volume = 10,
number = 1,
pages = "1--9",
year = 2020,
publisher = "Nature Publishing Group",
abstract = "Films of SiGe nanocrystals (NCs) in oxide have the advantage of tuning the energy band gap by adjusting SiGe NCs composition and size. In this study, SiGe-SiO2 amorphous films were deposited by magnetron sputtering on Si substrate followed by rapid thermal annealing at 700, 800 and 1000 °C. We investigated films with Si:Ge:SiO2 compositions of 25:25:50 vol.% and 5:45:50 vol.%. TEM investigations reveal the major changes in films morphology (SiGe NCs with different sizes and densities) produced by Si:Ge ratio and annealing temperature. XPS also show that the film depth profile of SiGe content is dependent on the annealing temperature. These changes strongly influence electrical and photoconduction properties. Depending on annealing temperature and Si:Ge ratio, photocurrents can be 103 times higher than dark currents. The photocurrent cutoff wavelength obtained on samples with 25:25 vol% SiGe ratio decreases with annealing temperature increase from 1260 nm in SWIR for 700 °C annealed films to 1210 nm for those at 1000 °C. By increasing Ge content in SiGe (5:45 vol%) the cutoff wavelength significantly shifts to 1345 nm (800 °C annealing). By performing measurements at 100 K, the cutoff wavelength extends in SWIR to 1630 nm having high photoresponsivity of 9.35 AW−1.",
doi = "10.1038/s41598-020-60000-x"
}

29. H V Haraldsson, K Torfason, A Manolescu and Á Valfells.
Molecular Dynamics Simulations of Mutual Space-Charge Effect Between Planar Field Emitters.
IEEE Transactions on Plasma Science , 1-7 (2020).
Abstract Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given.
DOI BibTeX

@article{9093900,
author = "H. V. {Haraldsson} and K. {Torfason} and A. {Manolescu} and Á. {Valfells}",
journal = "IEEE Transactions on Plasma Science",
title = "Molecular Dynamics Simulations of Mutual Space-Charge Effect Between Planar Field Emitters",
year = 2020,
volume = "",
number = "",
pages = "1-7",
doi = "10.1109/TPS.2020.2991582",
abstract = "Molecular dynamics simulations, with full Coulomb interaction and self-consistent field emission, are used to examine mutual space-charge interactions between beams originating from several emitter areas, in a planar infinite diode. The simulations allow observation of the trajectory of each individual electron through the diode gap. Results show that when the center-to-center spacing between emitters is greater than half of the gap spacing the emitters are essentially independent. For smaller spacing the mutual space-charge effect increases rapidly and should not be discounted. A simple qualitative explanation for this effect is given."
}

30. G A Nemnes, T L Mitran, A Manolescu and Daniela Dragoman.
Electric field effect in boron and nitrogen doped graphene bilayers.
Computational Materials Science 155, 175 - 179 (2018).
Abstract Unlike single layer graphene, in the case of AB-stacked bilayer graphene (BLG) one can induce a non-zero energy gap by breaking the inversion symmetry between the two layers using a perpendicular electric field. This is an essential requirement in field-effect applications, particularly since the induced gap in BLG systems can be further tuned by the magnitude of the external electric field. Doping is another way to modify the electronic properties of graphene based systems. We investigate here BLG systems doped with boron and nitrogen in the presence of external electric field, in the framework of density functional theory (DFT) calculations. Highly doped BLG systems are known to behave as degenerate semiconductors, where the Fermi energy depends on the doping concentration but, in addition, we show that the electronic properties drastically depend also on the applied electric field. By changing the magnitude and the orientation of the electric field, the gap size and position relative to the Fermi level may be tuned, essentially controlling the effect of the extrinsic doping. In this context, we discuss in how far the external electric field may suitably adjust the effective doping and, implicitly, the conduction properties of doped BLG systems.
URL, DOI BibTeX

@article{NEMNES2018175,
title = "Electric field effect in boron and nitrogen doped graphene bilayers",
journal = "Computational Materials Science",
volume = 155,
pages = "175 - 179",
year = 2018,
issn = "0927-0256",
doi = "https://doi.org/10.1016/j.commatsci.2018.08.054",
url = "http://www.sciencedirect.com/science/article/pii/S0927025618305822",
author = "G.A. Nemnes and T.L. Mitran and A. Manolescu and Daniela Dragoman",
keywords = "Graphene bilayer, Field effect, Effective doping",
abstract = "Unlike single layer graphene, in the case of AB-stacked bilayer graphene (BLG) one can induce a non-zero energy gap by breaking the inversion symmetry between the two layers using a perpendicular electric field. This is an essential requirement in field-effect applications, particularly since the induced gap in BLG systems can be further tuned by the magnitude of the external electric field. Doping is another way to modify the electronic properties of graphene based systems. We investigate here BLG systems doped with boron and nitrogen in the presence of external electric field, in the framework of density functional theory (DFT) calculations. Highly doped BLG systems are known to behave as degenerate semiconductors, where the Fermi energy depends on the doping concentration but, in addition, we show that the electronic properties drastically depend also on the applied electric field. By changing the magnitude and the orientation of the electric field, the gap size and position relative to the Fermi level may be tuned, essentially controlling the effect of the extrinsic doping. In this context, we discuss in how far the external electric field may suitably adjust the effective doping and, implicitly, the conduction properties of doped BLG systems."
}

31. M T Sultan, J T Gudmundsson, A Manolescu, M L Ciurea, C Palade, A V Maraloiu and H G Svavarsson.
Enhanced Photoconductivity of SIGE-Trilayer Stack by Retrenching Annealing Conditions.
In 2018 International Semiconductor Conference (CAS) (). (2018), 61-64.
BibTeX

@inproceedings{8539775,
author = "M. T. {Sultan} and J. T. {Gudmundsson} and A. {Manolescu} and M. L. {Ciurea} and C. {Palade} and A. V. {Maraloiu} and H. G. {Svavarsson}",
booktitle = "2018 International Semiconductor Conference (CAS)",
title = "Enhanced Photoconductivity of SIGE-Trilayer Stack by Retrenching Annealing Conditions",
year = 2018,
volume = "",
number = "",
pages = "61-64"
}

32. M T Sultan, J T Gudmundsson, A Manolescu, M L Ciurea and H G Svavarsson.
The Effect of H2/Ar Plasma Treatment Over Photoconductivity of Sige Nanoparticles Sandwiched Between Silicon Oxide Matrix.
In 2018 International Semiconductor Conference (CAS) (). (2018), 257-260.
BibTeX

@inproceedings{8539761,
author = "M. T. {Sultan} and J. T. {Gudmundsson} and A. {Manolescu} and M. L. {Ciurea} and H. G. {Svavarsson}",
booktitle = "2018 International Semiconductor Conference (CAS)",
title = "The Effect of H2/Ar Plasma Treatment Over Photoconductivity of Sige Nanoparticles Sandwiched Between Silicon Oxide Matrix",
year = 2018,
volume = "",
number = "",
pages = "257-260"
}

33. M U Torres, A Sitek, V Gudmundsson and A Manolescu.
Radiated fields by polygonal core-shell nanowires.
In 2018 20th International Conference on Transparent Optical Networks (ICTON) (). (2018), 1-4.
Abstract We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires.
arXiv BibTeX

@inproceedings{8473825,
author = "M. U. {Torres} and A. {Sitek} and V. {Gudmundsson} and A. {Manolescu}",
booktitle = "2018 20th International Conference on Transparent Optical Networks (ICTON)",
title = "Radiated fields by polygonal core-shell nanowires",
year = 2018,
volume = "",
number = "",
pages = "1-4",
arxiv = "https://arxiv.org/abs/1804.07959",
abstract = "We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires."
}

34. Tudor D Stanescu, Anna Sitek and Andrei Manolescu.
Robust topological phase in proximitized core–shell nanowires coupled to multiple superconductors.
Beilstein Journal of Nanotechnology 9, 1512-1526 (2018).
Abstract We consider core-shell nanowires with prismatic geometry contacted with two or more superconductors in the presence of a magnetic field applied parallel to the wire. In this geometry, the lowest energy states are localized on the outer edges of the shell, which strongly inhibits the orbital effects of the longitudinal magnetic field that are detrimental to Majorana physics. Using a tight-binding model of coupled parallel chains, we calculate the topological phase diagram of the hybrid system in the presence of non-vanishing transverse potentials and finite relative phases between the parent superconductors. We show that having finite relative phases strongly enhances the stability of the induced topological superconductivity over a significant range of chemical potentials and reduces the value of the critical field associated with the topological quantum phase transition.
arXiv, DOI BibTeX

@article{Stanescu2018,
author = "Tudor D. Stanescu and Anna Sitek and Andrei Manolescu",
title = "Robust topological phase in proximitized core–shell nanowires coupled to multiple superconductors",
journal = "Beilstein Journal of Nanotechnology",
year = 2018,
volume = 9,
pages = "1512-1526",
issn = "2190-4286",
doi = "10.3762/bjnano.9.142",
arxiv = "https://arxiv.org/abs/1804.05446",
abstract = "We consider core-shell nanowires with prismatic geometry contacted with two or more superconductors in the presence of a magnetic field applied parallel to the wire. In this geometry, the lowest energy states are localized on the outer edges of the shell, which strongly inhibits the orbital effects of the longitudinal magnetic field that are detrimental to Majorana physics. Using a tight-binding model of coupled parallel chains, we calculate the topological phase diagram of the hybrid system in the presence of non-vanishing transverse potentials and finite relative phases between the parent superconductors. We show that having finite relative phases strongly enhances the stability of the induced topological superconductivity over a significant range of chemical potentials and reduces the value of the critical field associated with the topological quantum phase transition."
}

35. Anna Sitek, Miguel Urbaneja Torres, Kristinn Torfason, Vidar Gudmundsson, Andrea Bertoni and Andrei Manolescu.
Excitons in Core-Shell Nanowires with Polygonal Cross Sections.
Nano Letters 18, 2581-2589 (2018).
Abstract The distinctive prismatic geometry of semiconductor coreshell nanowires leads to complex localization patterns of carriers. Here, we describe the formation of optically active in-gap excitonic states induced by the interplay between localization of carriers in the corners and their mutual Coulomb interaction. To compute the energy spectra and configurations of excitons created in the conductive shell, we use a multielectron numerical approach based on the exact solution of the multiparticle Hamiltonian for electrons in the valence and conduction bands, which includes the Coulomb interaction in a nonperturbative manner. We expose the formation of well-separated quasidegenerate levels, and focus on the implications of the electron localization in the corners or on the sides of triangular, square, and hexagonal cross sections. We obtain excitonic in-gap states associated with symmetrically distributed electrons in the spin singlet configuration. They acquire large contributions due to Coulomb interaction, and thus are shifted to much higher energies than other states corresponding to the conduction electron and the vacancy localized in the same corner. We compare the results of the multielectron method with those of an electronhole model, and we show that the latter does not reproduce the singlet excitonic states. We also obtain the exciton lifetime and explain selection rules which govern the recombination process.
URL arXiv, DOI BibTeX

@article{doi:10.1021/acs.nanolett.8b00309,
author = "Sitek, Anna and Urbaneja Torres, Miguel and Torfason, Kristinn and Gudmundsson, Vidar and Bertoni, Andrea and Manolescu, Andrei",
title = "Excitons in Core-Shell Nanowires with Polygonal Cross Sections",
journal = "Nano Letters",
volume = 18,
number = 4,
pages = "2581-2589",
year = 2018,
doi = "10.1021/acs.nanolett.8b00309",
note = "PMID: 29578727",
url = "https://doi.org/10.1021/acs.nanolett.8b00309",
eprint = "https://doi.org/10.1021/acs.nanolett.8b00309",
arxiv = "https://hdl.handle.net/20.500.11815/694",
abstract = "The distinctive prismatic geometry of semiconductor coreshell nanowires leads to complex localization patterns of carriers. Here, we describe the formation of optically active in-gap excitonic states induced by the interplay between localization of carriers in the corners and their mutual Coulomb interaction. To compute the energy spectra and configurations of excitons created in the conductive shell, we use a multielectron numerical approach based on the exact solution of the multiparticle Hamiltonian for electrons in the valence and conduction bands, which includes the Coulomb interaction in a nonperturbative manner. We expose the formation of well-separated quasidegenerate levels, and focus on the implications of the electron localization in the corners or on the sides of triangular, square, and hexagonal cross sections. We obtain excitonic in-gap states associated with symmetrically distributed electrons in the spin singlet configuration. They acquire large contributions due to Coulomb interaction, and thus are shifted to much higher energies than other states corresponding to the conduction electron and the vacancy localized in the same corner. We compare the results of the multielectron method with those of an electronhole model, and we show that the latter does not reproduce the singlet excitonic states. We also obtain the exciton lifetime and explain selection rules which govern the recombination process."
}

36. Nzar Rauf Abdullah, Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Photon-induced tunability of the thermospin current in a Rashba ring.
Journal of Physics: Condensed Matter 30, 145303 (2018).
Abstract The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spinorbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the AharonovCasher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.
URL arXiv BibTeX

@article{0953-8984-30-14-145303,
author = "Nzar Rauf Abdullah and Thorsten Arnold and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Photon-induced tunability of the thermospin current in a Rashba ring",
journal = "Journal of Physics: Condensed Matter",
volume = 30,
number = 14,
pages = 145303,
url = "http://stacks.iop.org/0953-8984/30/i=14/a=145303",
year = 2018,
arxiv = "https://arxiv.org/abs/1712.03386",
abstract = "The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spinorbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the AharonovCasher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field."
}

37. George Alexandru Nemnes, Cristina Besleaga, Andrei Gabriel Tomulescu, Alexandra Palici, Lucian Pintilie, Andrei Manolescu and Ioana Pintilie.
How measurement protocols influence the dynamic J-V characteristics of perovskite solar cells: Theory and experiment.
Solar Energy 173, 976 - 983 (2018).
Abstract The dynamic effects observed in the J-V measurements represent one important hallmark in the behavior of the perovskite solar cells. Proper measurement protocols (MPs) should be employed for the experimental data reproducibility, in particular for a reliable evaluation of the power conversion efficiency (PCE), as well as for a meaningful characterization of the type and magnitude of the hysteresis. We discuss here several MPs by comparing the experimental J-V characteristics with simulated ones using the dynamic electrical model (DEM). Pre-poling conditions and bias scan rate can have a dramatic influence not only on the apparent solar cell performance, but also on the hysteretic phenomena. Under certain measurement conditions, a hysteresis-free behavior with relatively high PCEs may be observed, although the J-V characteristics may be far away from the stationary case. Furthermore, forward-reverse and reverse-forward bias scans show qualitatively different behaviors regarding the type of the hysteresis, normal and inverted, depending on the bias pre-poling. We emphasize here that correlated double-scans, forward-reverse or reverse-forward, where the second scan is conducted in the opposite sweep direction and begins immediately after the first scan is complete, are essential for a correct assessment of the dynamic hysteresis. In this context, we define a hysteresis index which consistently assigns the hysteresis type and magnitude. Our DEM simulations, supported by experimental data, provide further guidance for an efficient and accurate determination of the stationary J-V characteristics, showing that the type and magnitude of the dynamic hysteresis may be affected by unintentional pre-conditioning in typical experiments.
URL arXiv, DOI BibTeX

@article{NEMNES2018976,
title = "How measurement protocols influence the dynamic J-V characteristics of perovskite solar cells: Theory and experiment",
journal = "Solar Energy",
volume = 173,
pages = "976 - 983",
year = 2018,
issn = "0038-092X",
doi = "https://doi.org/10.1016/j.solener.2018.08.033",
url = "http://www.sciencedirect.com/science/article/pii/S0038092X1830803X",
arxiv = "https://arxiv.org/abs/1803.00285",
author = "George Alexandru Nemnes and Cristina Besleaga and Andrei Gabriel Tomulescu and Alexandra Palici and Lucian Pintilie and Andrei Manolescu and Ioana Pintilie",
keywords = "Measurement protocol, Perovskite solar cell, Hysteresis type, Hysteresis index",
abstract = "The dynamic effects observed in the J-V measurements represent one important hallmark in the behavior of the perovskite solar cells. Proper measurement protocols (MPs) should be employed for the experimental data reproducibility, in particular for a reliable evaluation of the power conversion efficiency (PCE), as well as for a meaningful characterization of the type and magnitude of the hysteresis. We discuss here several MPs by comparing the experimental J-V characteristics with simulated ones using the dynamic electrical model (DEM). Pre-poling conditions and bias scan rate can have a dramatic influence not only on the apparent solar cell performance, but also on the hysteretic phenomena. Under certain measurement conditions, a hysteresis-free behavior with relatively high PCEs may be observed, although the J-V characteristics may be far away from the stationary case. Furthermore, forward-reverse and reverse-forward bias scans show qualitatively different behaviors regarding the type of the hysteresis, normal and inverted, depending on the bias pre-poling. We emphasize here that correlated double-scans, forward-reverse or reverse-forward, where the second scan is conducted in the opposite sweep direction and begins immediately after the first scan is complete, are essential for a correct assessment of the dynamic hysteresis. In this context, we define a hysteresis index which consistently assigns the hysteresis type and magnitude. Our DEM simulations, supported by experimental data, provide further guidance for an efficient and accurate determination of the stationary J-V characteristics, showing that the type and magnitude of the dynamic hysteresis may be affected by unintentional pre-conditioning in typical experiments."
}

38. Gudmundsson Vidar, Abdulla Nzar Rauf, Sitek Anna, Goan Hsi-Sheng, Tang Chi-Shung and Manolescu Andrei.
Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity.
Annalen der Physik 530, 1700334 (2017).
Abstract Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.
URL arXiv, DOI BibTeX

@article{doi10.1002andp201700334,
author = "Gudmundsson Vidar and Abdulla Nzar Rauf and Sitek Anna and Goan Hsi-Sheng and Tang Chi-Shung and Manolescu Andrei",
title = "Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity",
journal = "Annalen der Physik",
volume = 530,
number = 2,
year = 2017,
pages = 1700334,
keywords = "configuration interactions, electroluminescence, electron transport, photon cavity, photon correlations",
doi = "10.1002/andp.201700334",
url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.201700334",
eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201700334",
arxiv = "https://arxiv.org/abs/1706.03483",
abstract = "Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence."
}

39. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Spin-dependent heat and thermoelectric currents in a Rashba ring coupled to a photon cavity.
Physica E: Low-dimensional Systems and Nanostructures 95, 102 - 107 (2018).
Abstract Spin-dependent heat and thermoelectric currents in a quantum ring with Rashba spin-orbit interaction placed in a photon cavity are theoretically calculated. The quantum ring is coupled to two external leads with different temperatures. In a resonant regime, with the ring structure in resonance with the photon field, the heat and the thermoelectric currents can be controlled by the Rashba spin-orbit interaction. The heat current is suppressed in the presence of the photon field due to contribution of the two-electron and photon replica states to the transport while the thermoelectric current is not sensitive to changes in parameters of the photon field. Our study opens a possibility to use the proposed interferometric device as a tunable heat current generator in the cavity photon field.
URL arXiv, DOI BibTeX

@article{ABDULLAH2018102,
title = "Spin-dependent heat and thermoelectric currents in a Rashba ring coupled to a photon cavity",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 95,
pages = "102 - 107",
year = 2018,
issn = "1386-9477",
doi = "https://doi.org/10.1016/j.physe.2017.09.011",
url = "http://www.sciencedirect.com/science/article/pii/S1386947717311372",
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
keywords = "Thermo-optic effects, Electronic transport in mesoscopic systems, Cavity quantum electrodynamics, Electro-optical effects",
arxiv = "https://arxiv.org/abs/1707.08416",
abstract = "Spin-dependent heat and thermoelectric currents in a quantum ring with Rashba spin-orbit interaction placed in a photon cavity are theoretically calculated. The quantum ring is coupled to two external leads with different temperatures. In a resonant regime, with the ring structure in resonance with the photon field, the heat and the thermoelectric currents can be controlled by the Rashba spin-orbit interaction. The heat current is suppressed in the presence of the photon field due to contribution of the two-electron and photon replica states to the transport while the thermoelectric current is not sensitive to changes in parameters of the photon field. Our study opens a possibility to use the proposed interferometric device as a tunable heat current generator in the cavity photon field."
}

40. K Torfason, H V Haraldsson, Á Valfells and A Manolescu.
Molecular dynamics simulations of vacuum diodes.
In 2018 IEEE International Vacuum Electronics Conference (IVEC) (). (2018), 63-64.
BibTeX

@inproceedings{8391553,
author = "K. {Torfason} and H. V. {Haraldsson} and Á. {Valfells} and A. {Manolescu}",
booktitle = "2018 IEEE International Vacuum Electronics Conference (IVEC)",
title = "Molecular dynamics simulations of vacuum diodes",
year = 2018,
volume = "",
number = "",
pages = "63-64"
}

41. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Effects of photon field on heat transport through a quantum wire attached to leads.
Physics Letters A 382, 199 - 204 (2018).
Abstract We theoretically investigate photo-thermoelectric transport through a quantum wire in a photon cavity coupled to electron reservoirs with different temperatures. Our approach, based on a quantum master equation, allows us to investigate the influence of a quantized photon field on the heat current and thermoelectric transport in the system. We find that the heat current through the quantum wire is influenced by the photon field resulting in a negative heat current in certain cases. The characteristics of the transport are studied by tuning the ratio, h??/kB?T, between the photon energy, h??, and the thermal energy, kB?T. The thermoelectric transport is enhanced by the cavity photons when kB?T>h??. By contrast, if kB?T
URL arXiv, DOI BibTeX

@article{ABDULLAH2018199,
title = "Effects of photon field on heat transport through a quantum wire attached to leads",
journal = "Physics Letters A",
volume = 382,
number = 4,
pages = "199 - 204",
year = 2018,
issn = "0375-9601",
doi = "https://doi.org/10.1016/j.physleta.2017.11.007",
url = "http://www.sciencedirect.com/science/article/pii/S0375960117311209",
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
arxiv = "https://arxiv.org/abs/1711.01210",
keywords = "Thermo-optic effects, Electronic transport in mesoscopic systems, Cavity quantum electrodynamics, Electro-optical effects",
abstract = "We theoretically investigate photo-thermoelectric transport through a quantum wire in a photon cavity coupled to electron reservoirs with different temperatures. Our approach, based on a quantum master equation, allows us to investigate the influence of a quantized photon field on the heat current and thermoelectric transport in the system. We find that the heat current through the quantum wire is influenced by the photon field resulting in a negative heat current in certain cases. The characteristics of the transport are studied by tuning the ratio, h??/kB?T, between the photon energy, h??, and the thermal energy, kB?T. The thermoelectric transport is enhanced by the cavity photons when kB?T>h??. By contrast, if kB?T
42. Sigurdur I Erlingsson, Jens H Bardarson and Andrei Manolescu.
Thermoelectric current in topological insulator nanowires with impurities.
Beilstein journal of nanotechnology 9, 1156 (2018).
Abstract In this paper we consider charge current generated by maintaining a temperature difference over a nanowire at zero voltage bias. For topological insulator nanowires in a perpendicular magnetic field the current can change sign as the temperature of one end is increased. Here we study how this thermoelectric current sign reversal depends on the magnetic field and how impurities affect the size of the thermoelectric current. We consider both scalar and magnetic impurities and show that their influence on the current are quite similar, although the magnetic impurities seem to be more effective in reducing the effect. For moderate impurity concentration the sign reversal persists.
arXiv BibTeX

@article{erlingsson2018thermoelectric,
title = "Thermoelectric current in topological insulator nanowires with impurities",
author = "Erlingsson, Sigurdur I and Bardarson, Jens H and Manolescu, Andrei",
journal = "Beilstein journal of nanotechnology",
volume = 9,
pages = 1156,
year = 2018,
arxiv = "https://arxiv.org/abs/1803.04507",
publisher = "Beilstein-Institut",
abstract = "In this paper we consider charge current generated by maintaining a temperature difference over a nanowire at zero voltage bias. For topological insulator nanowires in a perpendicular magnetic field the current can change sign as the temperature of one end is increased. Here we study how this thermoelectric current sign reversal depends on the magnetic field and how impurities affect the size of the thermoelectric current. We consider both scalar and magnetic impurities and show that their influence on the current are quite similar, although the magnetic impurities seem to be more effective in reducing the effect. For moderate impurity concentration the sign reversal persists."
}

43. Miguel Urbaneja Torres, Anna Sitek, Vidar Gudmundsson and Andrei Manolescu.
Radiated fields by polygonal core-shell nanowires.
arXiv preprint arXiv:1804.07959 (2018).
Abstract We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires.
arXiv BibTeX

@article{torres2018radiated,
title = "Radiated fields by polygonal core-shell nanowires",
author = "Torres, Miguel Urbaneja and Sitek, Anna and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1804.07959",
arxiv = "https://arxiv.org/abs/1804.07959",
year = 2018,
abstract = "We calculate the electromagnetic field radiated by tubular nanowires with prismatic geometry and infinite length. The polygonal geometry has implications on the electronic localization; the lowest energy states are localized at the edges of the prism and are separated by a considerable energy gap from the states localized on the facets. This localization can be controlled with external electric or magnetic fields. In particular, by applying a magnetic field transverse to the wire the states may become localized on the lateral regions of the shell, relatively to the direction of the field, leading to channels of opposite currents. Because of the prismatic geometry of the nanowire the current distribution, and hence the radiated electromagnetic field, have an anisotropic structure, which can be modified by the external fields. In this work we study hexagonal, square and triangular nanowires."
}

44. Alexandra Palici, George Alexandru Nemnes, Cristina Besleaga, Lucian Pintilie, Dragos-Victor Anghel, Ioana Pintilie and Andrei Manolescu.
The Influence of the Relaxation Time on the Dynamic Hysteresis in Perovskite Solar Cells.
EPJ Web Conf. 173, 03017 (2018).
URL, DOI BibTeX

@article{refId0,
author = "{Palici, Alexandra} and {Nemnes, George Alexandru} and {Besleaga, Cristina} and {Pintilie, Lucian} and {Anghel, Dragos-Victor} and {Pintilie, Ioana} and {Manolescu, Andrei}",
title = "The Influence of the Relaxation Time on the Dynamic Hysteresis in Perovskite Solar Cells",
doi = "10.1051/epjconf/201817303017",
url = "https://doi.org/10.1051/epjconf/201817303017",
journal = "EPJ Web Conf.",
year = 2018,
volume = 173,
pages = 03017
}

45. Nzar Rauf Abdullah, Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Photon-induced tunability of the thermospin current in a Rashba ring.
Journal of Physics: Condensed Matter 30, 145303 (March 2018).
Abstract The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin–orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov–Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.
URL, DOI BibTeX

@article{Abdullah_2018,
doi = "10.1088/1361-648x/aab255",
url = "https://doi.org/10.1088%2F1361-648x%2Faab255",
year = 2018,
month = "mar",
publisher = "{IOP} Publishing",
volume = 30,
number = 14,
pages = 145303,
author = "Nzar Rauf Abdullah and Thorsten Arnold and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Photon-induced tunability of the thermospin current in a Rashba ring",
journal = "Journal of Physics: Condensed Matter",
abstract = "The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin–orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov–Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field."
}

46. Vidar Gudmundsson, Nzar Rauf Abdulla, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity.
Annalen der Physik 530, 1700334 (2018).
Abstract Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.
URL, DOI BibTeX

@article{doi:10.1002/andp.201700334,
author = "Gudmundsson, Vidar and Abdulla, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
title = "Electroluminescence Caused by the Transport of Interacting Electrons through Parallel Quantum Dots in a Photon Cavity",
journal = "Annalen der Physik",
volume = 530,
number = 2,
pages = 1700334,
keywords = "configuration interactions, electroluminescence, electron transport, photon cavity, photon correlations",
doi = "10.1002/andp.201700334",
url = "https://onlinelibrary.wiley.com/doi/abs/10.1002/andp.201700334",
eprint = "https://onlinelibrary.wiley.com/doi/pdf/10.1002/andp.201700334",
abstract = "Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.",
year = 2018
}

47. Vidar Gudmundsson, Thorsteinn H Jonsson, Maria Laura Bernodusson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state.
Annalen der Physik 529, 1600177–n/a (2017).
Abstract We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions.
URL arXiv, DOI BibTeX

@article{ANDP:ANDP201600177,
author = "Gudmundsson, Vidar and Jonsson, Thorsteinn H. and Bernodusson, Maria Laura and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
title = "Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state",
journal = "Annalen der Physik",
volume = 529,
number = "1-2",
issn = "1521-3889",
url = "http://dx.doi.org/10.1002/andp.201600177",
doi = "10.1002/andp.201600177",
pages = "1600177--n/a",
year = 2017,
note = 1600177,
arxiv = "https://arxiv.org/abs/1605.08248",
abstract = "We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions."
}

48. Razvan Chirla, Cristian-Dorin Horea, Traian-Octavian Costea, Radu Dragomir, Andrei Manolescu and Cătălin Paşcu Moca.
Shiba states coupled to a resonant cavity.
AIP Conference Proceedings 1796, 030002 (2017).
Abstract When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations.
URL, DOI BibTeX

@article{doi:10.1063/1.4972367,
author = "Razvan Chirla and Cristian-Dorin Horea and Traian-Octavian Costea and Radu Dragomir and Andrei Manolescu and Cătălin Paşcu Moca",
title = "Shiba states coupled to a resonant cavity",
journal = "AIP Conference Proceedings",
volume = 1796,
number = 1,
pages = 030002,
year = 2017,
doi = "10.1063/1.4972367",
url = "http://aip.scitation.org/doi/abs/10.1063/1.4972367",
eprint = "http://aip.scitation.org/doi/pdf/10.1063/1.4972367",
abstract = "When a quantum dot is embedded into a superconducting environment, it leads to the formations of localized Shiba states inside the gap. If the Coulomb interaction is sufficiently small, the Shiba states consist of a pair of singlet states and a doublet, that compete for the ground state and induce a quantum phase transition. In the presence of an external microwave field, the Shiba energy spectrum is significantly modified. Moreover, the transmission of the cavity inherits features that can pinpoint the exact location of the quantum critical point. In terms of methods used, our analytical calculations are supplemented by state of the art numerical renormalization group calculations."
}

49. Neculai Plugaru, George Alexandru Nemnes, Lucian Filip, Ioana Pintilie, Lucian Pintilie, Keith Tobias Butler and Andrei Manolescu.
Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces.
The Journal of Physical Chemistry C 121, 9096-9109 (2017).
Abstract The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles.
URL, DOI BibTeX

@article{doi:10.1021/acs.jpcc.7b00399,
author = "Plugaru, Neculai and Nemnes, George Alexandru and Filip, Lucian and Pintilie, Ioana and Pintilie, Lucian and Butler, Keith Tobias and Manolescu, Andrei",
title = "Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces",
journal = "The Journal of Physical Chemistry C",
volume = 121,
number = 17,
pages = "9096-9109",
year = 2017,
doi = "10.1021/acs.jpcc.7b00399",
url = "http://dx.doi.org/10.1021/acs.jpcc.7b00399",
eprint = "http://dx.doi.org/10.1021/acs.jpcc.7b00399",
abstract = "The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20\% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO internal electric field leads to a position dependent energy levels diagram. (iii) The HP gap may be tuned by chlorine concentration at the interface, as well as the by the surface terminations of PbTiO3 and hybrid perovskite layers. (iv) The presence of the PTO ferroelectric surface is likely to have just a slight orientational effect on the (CH3NH3)+ dipoles."
}

50. Anna Sitek, Miguel Urbaneja Torres, Kristinn Torfason, Vidar Gudmundsson and Andrei Manolescu.
Controlled Coulomb effects in core-shell quantum rings.
arXiv preprint arXiv:1704.06136 (2017).
Abstract We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields.
arXiv BibTeX

@article{sitek2017controlled,
title = "Controlled Coulomb effects in core-shell quantum rings",
author = "Sitek, Anna and Torres, Miguel Urbaneja and Torfason, Kristinn and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1704.06136",
year = 2017,
arxiv = "https://arxiv.org/abs/1704.06136",
abstract = "We analyse theoretically the possibilities of contactless control of in-gap states formed by a pair of electrons confined in a triangular quantum ring. The in-gap states are corner-localized states associated with two electrons occupying the same corner area, and thus shifted to much higher energies than other corner states, but still they are below the energies of corner-side-localized states. We show how the energies, degeneracy and splittings between consecutive levels change with the orientation of an external electric field relatively to the polygonal cross section. We also show how absorption changes in the presence of external electric and magnetic fields."
}

51. G A Nemnes, Camelia Visan and A Manolescu.
Electronic and thermal conduction properties of halogenated porous graphene nanoribbons.
J. Mater. Chem. C 5, 4435-4441 (2017).
Abstract We investigate the electronic and thermal properties of porous graphene (PG) structures passivated with halogen atoms as possible candidates for efficient thermoelectric devices in the framework of density functional theory (DFT) calculations. Armchair and zigzag halogenated PG nanoribbons are analyzed comparatively. The electronic properties are consistent with the expected behavior for the two types of terminations, however with marked influences introduced by the different halogen atoms. Depending on the pore sizes and halogen type pseudo-gaps in the phononic band structure are visible in the low frequency range, which are particularly important for the thermal conduction at low temperatures. The gaps are systematically displaced towards lower energies as the atomic number of the halogen increases. At the same time, the electronic gap decreases, which is also essential for attaining a large figure of merit in a thermoelectric device. This opens the possibility of tuning both electronic and thermal properties of PG structures by halogen passivation.
URL arXiv, DOI BibTeX

@article{C7TC00029D,
author = "Nemnes, G. A. and Visan, Camelia and Manolescu, A.",
title = "Electronic and thermal conduction properties of halogenated porous graphene nanoribbons",
journal = "J. Mater. Chem. C",
year = 2017,
volume = 5,
issue = 18,
pages = "4435-4441",
publisher = "The Royal Society of Chemistry",
doi = "10.1039/C7TC00029D",
url = "http://dx.doi.org/10.1039/C7TC00029D",
arxiv = "https://arxiv.org/abs/1612.07949",
abstract = "We investigate the electronic and thermal properties of porous graphene (PG) structures passivated with halogen atoms as possible candidates for efficient thermoelectric devices in the framework of density functional theory (DFT) calculations. Armchair and zigzag halogenated PG nanoribbons are analyzed comparatively. The electronic properties are consistent with the expected behavior for the two types of terminations{,} however with marked influences introduced by the different halogen atoms. Depending on the pore sizes and halogen type pseudo-gaps in the phononic band structure are visible in the low frequency range{,} which are particularly important for the thermal conduction at low temperatures. The gaps are systematically displaced towards lower energies as the atomic number of the halogen increases. At the same time{,} the electronic gap decreases{,} which is also essential for attaining a large figure of merit in a thermoelectric device. This opens the possibility of tuning both electronic and thermal properties of PG structures by halogen passivation."
}

52. George Alexandru Nemnes, Cristina Besleaga, Viorica Stancu, Daniela Emilia Dogaru, Lucia Nicoleta Leonat, Lucian Pintilie, Kristinn Torfason, Marjan Ilkov, Andrei Manolescu and Ioana Pintilie.
Normal and Inverted Hysteresis in Perovskite Solar Cells.
The Journal of Physical Chemistry C 121, 11207-11214 (2017).
Abstract Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and possible solutions for a correct photoconversion efficiency evaluation.
URL, DOI BibTeX

@article{doi:10.1021/acs.jpcc.7b04248,
author = "Nemnes, George Alexandru and Besleaga, Cristina and Stancu, Viorica and Dogaru, Daniela Emilia and Leonat, Lucia Nicoleta and Pintilie, Lucian and Torfason, Kristinn and Ilkov, Marjan and Manolescu, Andrei and Pintilie, Ioana",
title = "Normal and Inverted Hysteresis in Perovskite Solar Cells",
journal = "The Journal of Physical Chemistry C",
volume = 121,
number = 21,
pages = "11207-11214",
year = 2017,
doi = "10.1021/acs.jpcc.7b04248",
url = "http://dx.doi.org/10.1021/acs.jpcc.7b04248",
eprint = "http://dx.doi.org/10.1021/acs.jpcc.7b04248",
abstract = "Hysteretic effects are investigated in perovskite solar cells in the standard FTO/TiO2/CH3NH3PbI3–xClx/spiro-OMeTAD/Au configuration. We report normal (NH) and inverted hysteresis (IH) in the J–V characteristics occurring for the same device structure, and the behavior strictly depends on the prepoling bias. NH typically appears at prepoling biases larger than the open circuit bias, while pronounced IH occurs for negative bias prepoling. The transition from NH to IH is marked by an intermediate mixed hysteresis behavior characterized by a crossing point in the J–V characteristics. The measured J–V characteristics are explained quantitatively by the dynamic electrical model. Furthermore, the influence of the bias scan rate on the NH/IH hysteresis is discussed based on the time evolution of the accumulated ionic and electronic polarization charge at the interfaces. Introducing a three-step measurement protocol, which includes stabilization, prepoling, and measurement, we put forward the difficulties and possible solutions for a correct photoconversion efficiency evaluation."
}

53. Andrei Manolescu, Anna Sitek, Javier Osca, Lloren$ç$ Serra, Vidar Gudmundsson and Tudor Dan Stanescu.
Majorana states in prismatic core-shell nanowires.
Phys. Rev. B 96, 125435 (September 2017).
Abstract We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell.
URL arXiv, DOI BibTeX

@article{PhysRevB.96.125435,
title = "Majorana states in prismatic core-shell nanowires",
author = "Manolescu, Andrei and Sitek, Anna and Osca, Javier and Serra, Lloren{$\c{c}$} and Gudmundsson, Vidar and Stanescu, Tudor Dan",
journal = "Phys. Rev. B",
volume = 96,
issue = 12,
pages = 125435,
numpages = 13,
year = 2017,
month = "Sep",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.96.125435",
arxiv = "https://arxiv.org/abs/1705.04950",
abstract = "We consider core-shell nanowires with conductive shell and insulating core and with polygonal cross section. We investigate the implications of this geometry on Majorana states expected in the presence of proximity-induced superconductivity and an external magnetic field. A typical prismatic nanowire has a hexagonal profile, but square and triangular shapes can also be obtained. The low-energy states are localized at the corners of the cross section, i.e., along the prism edges, and are separated by a gap from higher energy states localized on the sides. The corner localization depends on the details of the shell geometry, i.e., thickness, diameter, and sharpness of the corners. We study systematically the low-energy spectrum of prismatic shells using numerical methods and derive the topological phase diagram as a function of magnetic field and chemical potential for triangular, square, and hexagonal geometries. A strong corner localization enhances the stability of Majorana modes to various perturbations, including the orbital effect of the magnetic field, whereas a weaker localization favorizes orbital effects and reduces the critical magnetic field. The prismatic geometry allows the Majorana zero-energy modes to be accompanied by low-energy states, which we call pseudo Majorana, and which converge to real Majoranas in the limit of small shell thickness. We include the Rashba spin-orbit coupling in a phenomenological manner, assuming a radial electric field across the shell."
}

54. Vidar Gudmundsson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Time-dependent current into and through multilevel parallel quantum dots in a photon cavity.
Phys. Rev. B 95, 195307 (May 2017).
Abstract We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity.
URL arXiv, DOI BibTeX

@article{PhysRevB.95.195307,
title = "Time-dependent current into and through multilevel parallel quantum dots in a photon cavity",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 95,
issue = 19,
pages = 195307,
numpages = 12,
year = 2017,
month = "May",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.95.195307",
arxiv = "https://arxiv.org/abs/1611.09453",
abstract = "We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots embedded in a short wire placed in a photon cavity. A plunger gate is used to place specific many-body states of the interacting system in the bias window defined by the external leads. We show how the transport phenomena active in the many-level complex central system strongly depend on the gate voltage. We identify a resonant transport through the central system as the two spin components of the one-electron ground state are in the bias window. This resonant transport through the lowest energy electron states seems to a large extent independent of the detuned photon field when judged from the transport current. This could be expected in the small bias regime, but an observation of the occupancy of the states of the system reveals that this picture is not entirely true. The current does not reflect slower photon-active internal transitions bringing the system into the steady state. The number of initially present photons determines when the system reaches the real steady state. With two-electron states in the bias window we observe a more complex situation with intermediate radiative and nonradiative relaxation channels leading to a steady state with a weak nonresonant current caused by inelastic tunneling through the two-electron ground state of the system. The presence of the radiative channels makes this phenomena dependent on the number of photons initially in the cavity."
}

55. Vidar Gudmundsson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Electroluminescence caused by the transport of interacting electrons through parallel quantum dots in a photon cavity.
arXiv preprint arXiv:1706.03483 (2017).
Abstract We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence.
arXiv BibTeX

@article{gudmundsson2017electroluminescence,
title = "Electroluminescence caused by the transport of interacting electrons through parallel quantum dots in a photon cavity",
author = "Gudmundsson, Vidar and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1706.03483",
year = 2017,
arxiv = "https://arxiv.org/abs/1706.03483",
abstract = "We show that a Rabi-splitting of the states of strongly interacting electrons in parallel quantum dots embedded in a short quantum wire placed in a photon cavity can be produced by either the para- or the dia-magnetic electron-photon interactions when the geometry of the system is properly accounted for and the photon field is tuned close to a resonance with the electron system. We use these two resonances to explore the electroluminescence caused by the transport of electrons through the one- and two-electron ground states of the system and their corresponding conventional and vacuum electroluminescense as the central system is opened up by coupling it to external leads acting as electron reservoirs. Our analysis indicates that high-order electron-photon processes are necessary to adequately construct the cavity-photon dressed electron states needed to describe both types of electroluminescence."
}

56. Miguel Urbaneja Torres, Anna Sitek, Sigurdur I Erlingsson, Gunnar Thorgilsson, Vidar Gudmundsson and Andrei Manolescu.
Conductance features of core-shell nanowires determined by their internal geometry.
Phys. Rev. B 98, 085419 (August 2018).
URL, DOI BibTeX

@article{PhysRevB.98.085419,
title = "Conductance features of core-shell nanowires determined by their internal geometry",
author = "Urbaneja Torres, Miguel and Sitek, Anna and Erlingsson, Sigurdur I. and Thorgilsson, Gunnar and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 98,
issue = 8,
pages = 085419,
numpages = 10,
year = 2018,
month = "Aug",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.98.085419",
}

57. Sigurdur I Erlingsson, Andrei Manolescu, George Alexandru Nemnes, Jens H Bardarson and David Sanchez.
Reversal of Thermoelectric Current in Tubular Nanowires.
Phys. Rev. Lett. 119, 036804 (July 2017).
Abstract We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires.
URL arXiv, DOI BibTeX

@article{PhysRevLett.119.036804,
title = "Reversal of Thermoelectric Current in Tubular Nanowires",
author = "Erlingsson, Sigurdur I. and Manolescu, Andrei and Nemnes, George Alexandru and Bardarson, Jens H. and Sanchez, David",
journal = "Phys. Rev. Lett.",
volume = 119,
issue = 3,
pages = 036804,
numpages = 6,
year = 2017,
month = "Jul",
publisher = "American Physical Society",
doi = "10.1103/PhysRevLett.119.036804",
arxiv = "https://arxiv.org/abs/1705.01569",
abstract = "We calculate the charge current generated by a temperature bias between the two ends of a tubular nanowire. We show that in the presence of a transversal magnetic field the current can change sign; i.e., electrons can either flow from the hot to the cold reservoir, or in the opposite direction, when the temperature bias increases. This behavior occurs when the magnetic field is sufficiently strong, such that Landau and snaking states are created, and the energy dispersion is nonmonotonic with respect to the longitudinal wave vector. The sign reversal can survive in the presence of impurities. We predict this result for core-shell nanowires, for uniform nanowires with surface states due to the Fermi level pinning, and for topological insulator nanowires."
}

58. K Torfason, A Manolescu and Á Valfells.
Modelling Nano- and Microscale Vacuum Electronics A molecular dynamics approach.
In 2018 IEEE MTT-S International Conference on Numerical Electromagnetic and Multiphysics Modeling and Optimization (NEMO) (). (2018), 1-3.
BibTeX

@inproceedings{8503192,
author = "K. {Torfason} and A. {Manolescu} and Á. {Valfells}",
booktitle = "2018 IEEE MTT-S International Conference on Numerical Electromagnetic and Multiphysics Modeling and Optimization (NEMO)",
title = "Modelling Nano- and Microscale Vacuum Electronics A molecular dynamics approach",
year = 2018,
volume = "",
number = "",
pages = "1-3"
}

59. K Torfason, A Manolescu and Á Valfells.
High-fidelity Molecular Dynamics of Vacuum Nanoelectronics.
In 2018 31st International Vacuum Nanoelectronics Conference (IVNC) (). (2018), 1-2.
BibTeX

@inproceedings{8520287,
author = "K. {Torfason} and A. {Manolescu} and Á. {Valfells}",
booktitle = "2018 31st International Vacuum Nanoelectronics Conference (IVNC)",
title = "High-fidelity Molecular Dynamics of Vacuum Nanoelectronics",
year = 2018,
volume = "",
number = "",
pages = "1-2"
}

60. Vidar Gudmundsson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Current correlations for the transport of interacting electrons through parallel quantum dots in a photon cavity.
Physics Letters A 382, 1672 - 1678 (2018).
Abstract We calculate the current correlations for the steady-state electron transport through multi-level parallel quantum dots embedded in a short quantum wire, that is placed in a non-perfect photon cavity. We account for the electron–electron Coulomb interaction, and the para- and diamagnetic electron–photon interactions with a stepwise scheme of configuration interactions and truncation of the many-body Fock spaces. In the spectral density of the temporal current–current correlations we identify all the transitions, radiative and non-radiative, active in the system in order to maintain the steady state. We observe strong signs of two types of Rabi oscillations.
URL arXiv, DOI BibTeX

@article{GUDMUNDSSON20181672,
title = "Current correlations for the transport of interacting electrons through parallel quantum dots in a photon cavity",
journal = "Physics Letters A",
volume = 382,
number = 25,
pages = "1672 - 1678",
year = 2018,
issn = "0375-9601",
arxiv = "https://arxiv.org/abs/1707.08295",
doi = "https://doi.org/10.1016/j.physleta.2018.04.017",
url = "http://www.sciencedirect.com/science/article/pii/S0375960118303748",
author = "Vidar Gudmundsson and Nzar Rauf Abdullah and Anna Sitek and Hsi-Sheng Goan and Chi-Shung Tang and Andrei Manolescu",
keywords = "Current correlations, Electronic transport in mesoscopic systems, Cavity quantum electrodynamics, Electro-optical effects",
abstract = "We calculate the current correlations for the steady-state electron transport through multi-level parallel quantum dots embedded in a short quantum wire, that is placed in a non-perfect photon cavity. We account for the electron–electron Coulomb interaction, and the para- and diamagnetic electron–photon interactions with a stepwise scheme of configuration interactions and truncation of the many-body Fock spaces. In the spectral density of the temporal current–current correlations we identify all the transitions, radiative and non-radiative, active in the system in order to maintain the steady state. We observe strong signs of two types of Rabi oscillations."
}

61. Gunnar Thorgilsson, Sigurdur I Erlingsson and Andrei Manolescu.
Thermoelectric current in tubular nanowires in transverse electric and magnetic fields.
Journal of Physics: Conference Series 906, 012021 (2017).
Abstract In the presence of a transverse magnetic field, the charge current in nanowires can flow from the hot to the cold reservoir, but also backwards. The sign change can be obtained by increasing the temperature bias or the magnetic field. This behavior occurs when the magnetic field is sufficiently strong. Here, we will investigate how the size of the anomalous backward-flowing current is affected by an electric field perpendicular to the nanowire. The interplay of the electric and magnetic field modifies the dispersion curves, which will show up in the transport properties. We will also investigate how the presence of impurities affects the anomalous current. The electric field affects backscattering due to impurities, and thus the thermoelectric current reversal. Preliminary results show that the current reversal can survive in the presence of impurities.
URL BibTeX

@article{1742-6596-906-1-012021,
author = "Gunnar Thorgilsson and Sigurdur I. Erlingsson and Andrei Manolescu",
title = "Thermoelectric current in tubular nanowires in transverse electric and magnetic fields",
journal = "Journal of Physics: Conference Series",
volume = 906,
number = 1,
pages = 012021,
url = "http://stacks.iop.org/1742-6596/906/i=1/a=012021",
year = 2017,
abstract = "In the presence of a transverse magnetic field, the charge current in nanowires can flow from the hot to the cold reservoir, but also backwards. The sign change can be obtained by increasing the temperature bias or the magnetic field. This behavior occurs when the magnetic field is sufficiently strong. Here, we will investigate how the size of the anomalous backward-flowing current is affected by an electric field perpendicular to the nanowire. The interplay of the electric and magnetic field modifies the dispersion curves, which will show up in the transport properties. We will also investigate how the presence of impurities affects the anomalous current. The electric field affects backscattering due to impurities, and thus the thermoelectric current reversal. Preliminary results show that the current reversal can survive in the presence of impurities."
}

62. M Ni{ţ\ua}, M Ţolea, D C Marinescu and A Manolescu.
Hund and anti-Hund rules in circular molecules.
Phys. Rev. B 96, 235101 (December 2017).
Abstract We study the validity of Hund's first rule for the spin multiplicity in circular molecules—made of real or artificial atoms such as quantum dots—by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction.
URL, DOI BibTeX

@article{PhysRevB.96.235101,
title = "Hund and anti-Hund rules in circular molecules",
author = "Ni{\c{t}\u{a}}, M. and {\c{T}}olea, M. and Marinescu, D. C. and Manolescu, A.",
journal = "Phys. Rev. B",
volume = 96,
issue = 23,
pages = 235101,
numpages = 7,
year = 2017,
month = "Dec",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.96.235101",
abstract = "We study the validity of Hund's first rule for the spin multiplicity in circular molecules—made of real or artificial atoms such as quantum dots—by considering a perturbative approach in the Coulomb interaction in the extended Hubbard model with both on-site and long-range interactions. In this approximation, we show that an anti-Hund rule always defines the ground state in a molecule with 4N atoms at half-filling. In all other cases (i.e., number of atoms not a multiple of four, or a 4N molecule away from half-filling) both the singlet and the triplet outcomes are possible, as determined primarily by the total number of electrons in the system. In some instances, the Hund rule is always obeyed and the triplet ground state is realized mathematically for any values of the on-site and long-range interactions, while for other filling situations the singlet is also possible but only if the long-range interactions exceed a certain threshold, relatively to the on-site interaction."
}

63. Nzar Rauf Abdullah, Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Photon-induced tunability of the thermospin current in a Rashba ring.
arXiv preprint arXiv:1712.03386 (2017).
Abstract The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin-orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov-Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field.
arXiv BibTeX

@article{abdullah2017photon,
title = "Photon-induced tunability of the thermospin current in a Rashba ring",
author = "Abdullah, Nzar Rauf and Arnold, Thorsten and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "arXiv preprint arXiv:1712.03386",
year = 2017,
arxiv = "https://arxiv.org/abs/1712.03386",
abstract = "The goal of this work is to show how the thermospin polarization current in a quantum ring changes in the presence of Rashba spin-orbit coupling and a quantized single photon mode of a cavity the ring is placed in. Employing the reduced density operator and a general master equation formalism, we find that both the Rashba interaction and the photon field can significantly modulate the spin polarization and the thermospin polarization current. Tuning the Rashba coupling constant, degenerate energy levels are formed corresponding to the Aharonov-Casher destructive phase interference in the quantum ring system. Our analysis indicates that the maximum spin polarization can be observed at the points of degenerate energy levels due to spin accumulation in the system without the photon field. The thermospin current is thus suppressed. In the presence of the cavity, the photon field leads to an additional kinetic momentum of the electron. As a result the spin polarization can be enhanced by the photon field."
}

64. G A Nemnes, Cristina Besleaga, A G Tomulescu, Ioana Pintilie, L Pintilie, K Torfason and A Manolescu.
Dynamic electrical behavior of halide perovskite based solar cells.
Solar Energy Materials and Solar Cells 159, 197 - 203 (2017).
Abstract Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.
URL arXiv, DOI BibTeX

@article{NEMNES2017197,
title = "Dynamic electrical behavior of halide perovskite based solar cells",
journal = "Solar Energy Materials and Solar Cells",
volume = 159,
pages = "197 - 203",
year = 2017,
issn = "0927-0248",
doi = "https://doi.org/10.1016/j.solmat.2016.09.012",
url = "http://www.sciencedirect.com/science/article/pii/S0927024816303531",
author = "G.A. Nemnes and Cristina Besleaga and A.G. Tomulescu and Ioana Pintilie and L. Pintilie and K. Torfason and A. Manolescu",
keywords = "Perovskite solar cell, Hysteresis, Dynamic electrical model",
arxiv = "http://arxiv.org/abs/1606.00335",
abstract = "Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the J-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured J-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current overshoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions."
}

65. Anna Sitek, Mugurel Tolea, Marian Nita, Lloren$ç$ Serra, Vidar Gudmundsson and Andrei Manolescu.
In-gap corner states in core-shell polygonal quantum rings.
Scientific Reports 7, 40197 (January 2017).
Abstract We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states.
arXiv, DOI BibTeX

@article{sitek2017gap,
title = "In-gap corner states in core-shell polygonal quantum rings",
author = "Sitek, Anna and Tolea, Mugurel and Nita, Marian and Serra, Lloren{$\c{c}$} and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "Scientific Reports",
volume = 7,
pages = 40197,
arxiv = "http://arxiv.org/abs/1607.02107",
year = 2017,
month = "Jan",
doi = "10.1038/srep40197",
abstract = "We study Coulomb interacting electrons confined in polygonal quantum rings. We focus on the interplay of localization at the polygon corners and Coulomb repulsion. Remarkably, the Coulomb repulsion allows the formation of in-gap states, i.e., corner-localized states of electron pairs or clusters shifted to energies that were forbidden for non-interacting electrons, but below the energies of corner-side-localized states. We specify conditions allowing optical excitation to those states."
}

66. Kristinn Torfason, Agust Valfells and Andrei Manolescu.
Molecular Dynamics Simulations of Field Emission From a Prolate Spheroidal Tip.
arXiv preprint arXiv:1608.06789 (2016).
Abstract High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission from a prolate spheroidal tip. The space charge limited current is several times lower than the current calculated with the Fowler-Nordheim formula. The image-charge is taken into account with a spherical approximation, which is good around the top of the tip, i.e. region where the current is generated.
arXiv BibTeX

@article{torfason2016molecular,
title = "Molecular Dynamics Simulations of Field Emission From a Prolate Spheroidal Tip",
author = "Torfason, Kristinn and Valfells, Agust and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1608.06789",
arxiv = "http://arxiv.org/abs/1608.06789",
year = 2016,
abstract = "High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission from a prolate spheroidal tip. The space charge limited current is several times lower than the current calculated with the Fowler-Nordheim formula. The image-charge is taken into account with a spherical approximation, which is good around the top of the tip, i.e. region where the current is generated."
}

67. S Heedt, A Manolescu, G A Nemnes, W Prost, J Schubert, D Grützmacher and Th. Schäpers.
Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register.
Nano Letters 16, 4569-4575 (2016).
Abstract We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov–Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov–de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization.
URL, DOI BibTeX

@article{doi:10.1021/acs.nanolett.6b01840,
author = "S. Heedt and A. Manolescu and G. A. Nemnes and W. Prost and J. Schubert and D. Grützmacher and Th. Schäpers",
title = "Adiabatic Edge Channel Transport in a Nanowire Quantum Point Contact Register",
journal = "Nano Letters",
volume = 16,
number = 7,
pages = "4569-4575",
year = 2016,
doi = "10.1021/acs.nanolett.6b01840",
note = "PMID: 27347816",
url = "http://dx.doi.org/10.1021/acs.nanolett.6b01840",
eprint = "http://dx.doi.org/10.1021/acs.nanolett.6b01840",
abstract = "We report on a prototype device geometry where a number of quantum point contacts are connected in series in a single quasi-ballistic InAs nanowire. At finite magnetic field the backscattering length is increased up to the micron-scale and the quantum point contacts are connected adiabatically. Hence, several input gates can control the outcome of a ballistic logic operation. The absence of backscattering is explained in terms of selective population of spatially separated edge channels. Evidence is provided by regular Aharonov–Bohm-type conductance oscillations in transverse magnetic fields, in agreement with magnetoconductance calculations. The observation of the Shubnikov–de Haas effect at large magnetic fields corroborates the existence of spatially separated edge channels and provides a new means for nanowire characterization."
}

68. George Alexandru Nemnes, Cristina Besleaga, Andrei Gabriel Tomulescu, Ioana Pintilie, Lucian Pintilie, Kristinn Torfason and Andrei Manolescu.
Dynamic electrical behavior of halide perovskite based solar cells.
arXiv preprint arXiv:1606.00335 (2016).
Abstract A dynamic electrical model is introduced to investigate the hysteretic effects in the I-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured I-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current over-shoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions.
arXiv BibTeX

@article{nemnes2016dynamic,
title = "Dynamic electrical behavior of halide perovskite based solar cells",
author = "Nemnes, George Alexandru and Besleaga, Cristina and Tomulescu, Andrei Gabriel and Pintilie, Ioana and Pintilie, Lucian and Torfason, Kristinn and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1606.00335",
arxiv = "http://arxiv.org/abs/1606.00335",
year = 2016,
abstract = "A dynamic electrical model is introduced to investigate the hysteretic effects in the I-V characteristics of perovskite based solar cells. By making a simple ansatz for the polarization relaxation, our model is able to reproduce qualitatively and quantitatively detailed features of measured I-V characteristics. Pre-poling effects are discussed, pointing out the differences between initially over- and under-polarized samples. In particular, the presence of the current over-shoot observed in the reverse characteristics is correlated with the solar cell pre-conditioning. Furthermore, the dynamic hysteresis is analyzed with respect to changing the bias scan rate, the obtained results being consistent with experimentally reported data: the hysteresis amplitude is maximum at intermediate scan rates, while at very slow and very fast ones it becomes negligible. The effects induced by different relaxation time scales are assessed. The proposed dynamic electrical model offers a comprehensive view of the solar cell operation, being a practical tool for future calibration of tentative microscopic descriptions."
}

69. Vidar Gudmundsson, Thorsteinn H Jonsson, Maria Laura Bernodusson, Nzar Rauf Abdullah, Anna Sitek, Hsi-Sheng Goan, Chi-Shung Tang and Andrei Manolescu.
Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state.
arXiv preprint arXiv:1605.08248 (2016).
Abstract We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions.
arXiv BibTeX

@article{gudmundsson2016regimes,
title = "Regimes of radiative and nonradiative transitions in transport through an electronic system in a photon cavity reaching a steady state",
author = "Gudmundsson, Vidar and Jonsson, Thorsteinn H and Bernodusson, Maria Laura and Abdullah, Nzar Rauf and Sitek, Anna and Goan, Hsi-Sheng and Tang, Chi-Shung and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1605.08248",
arxiv = "http://arxiv.org/abs/1605.08248",
year = 2016,
abstract = "We analyze how a multilevel many-electron system in a photon cavity approaches the steady state when coupled to external leads. When a plunger gate is used to lower cavity photon dressed one- and two-electron states below the bias window defined by the external leads, we can identify one regime with nonradiative transitions dominating the electron transport, and another regime with radiative transitions. Both transitions trap the electrons in the states below the bias bringing the system into a steady state. The order of the two regimes and their relative strength depends on the location of the bias window in the energy spectrum of the system and the initial conditions."
}

70. Jeremy Capps, D C Marinescu and Andrei Manolescu.
Spin Seebeck effect in an (In,Ga)As quantum well with equal Rashba and Dresselhaus spin-orbit couplings.
Phys. Rev. B 93, 085307 (February 2016).
Abstract We demonstrate that a spin-dependent Seebeck effect can be detected in quantum wells with zinc-blend structure with equal Rashba-Dresselhaus spin-orbit couplings. This theory is based on the establishment of an itinerant antiferromagnetic state, a low total-energy configuration realized in the presence of the Coulomb interaction enabled by the k=0 degeneracy of the opposite-spin single-particle energy spectra. Transport in this state is modeled by using the solutions of a Boltzmann equation obtained within the relaxation time approximation. Numerical estimates performed for realistic GaAs samples indicate that at low temperatures, the amplitude of the spin Seebeck coefficient can be increased by scattering on magnetic impurities.
URL, DOI BibTeX

@article{PhysRevB.93.085307,
title = "Spin Seebeck effect in an (In,Ga)As quantum well with equal Rashba and Dresselhaus spin-orbit couplings",
author = "Capps, Jeremy and Marinescu, D. C. and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 93,
issue = 8,
pages = 085307,
numpages = 6,
year = 2016,
month = "Feb",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.93.085307",
abstract = "We demonstrate that a spin-dependent Seebeck effect can be detected in quantum wells with zinc-blend structure with equal Rashba-Dresselhaus spin-orbit couplings. This theory is based on the establishment of an itinerant antiferromagnetic state, a low total-energy configuration realized in the presence of the Coulomb interaction enabled by the k=0 degeneracy of the opposite-spin single-particle energy spectra. Transport in this state is modeled by using the solutions of a Boltzmann equation obtained within the relaxation time approximation. Numerical estimates performed for realistic GaAs samples indicate that at low temperatures, the amplitude of the spin Seebeck coefficient can be increased by scattering on magnetic impurities."
}

71. Andrei Manolescu, George Alexandru Nemnes, Anna Sitek, Tomas Orn Rosdahl, Sigurdur Ingi Erlingsson and Vidar Gudmundsson.
Conductance oscillations of core-shell nanowires in transversal magnetic fields.
Phys. Rev. B 93, 205445 (May 2016).
Abstract We analyze theoretically electronic transport through a core-shell nanowire in the presence of a transversal magnetic field. We calculate the conductance for a variable coupling between the nanowire and the attached leads and show how the snaking states, which are low-energy states localized along the lines of the vanishing radial component of the magnetic field, manifest their existence. In the strong-coupling regime they induce flux periodic, Aharonov-Bohm-like, conductance oscillations, which, by decreasing the coupling to the leads, evolve into well-resolved peaks. The flux periodic oscillations arise due to interference of the snaking states, which is a consequence of backscattering at either the contacts with leads or magnetic or potential barriers in the wire.
URL arXiv, DOI BibTeX

@article{PhysRevB.93.205445,
title = "Conductance oscillations of core-shell nanowires in transversal magnetic fields",
author = "Manolescu, Andrei and Nemnes, George Alexandru and Sitek, Anna and Rosdahl, Tomas Orn and Erlingsson, Sigurdur Ingi and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 93,
issue = 20,
pages = 205445,
numpages = 6,
year = 2016,
month = "May",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.93.205445",
arxiv = "http://arxiv.org/abs/1601.01477",
abstract = "We analyze theoretically electronic transport through a core-shell nanowire in the presence of a transversal magnetic field. We calculate the conductance for a variable coupling between the nanowire and the attached leads and show how the snaking states, which are low-energy states localized along the lines of the vanishing radial component of the magnetic field, manifest their existence. In the strong-coupling regime they induce flux periodic, Aharonov-Bohm-like, conductance oscillations, which, by decreasing the coupling to the leads, evolve into well-resolved peaks. The flux periodic oscillations arise due to interference of the snaking states, which is a consequence of backscattering at either the contacts with leads or magnetic or potential barriers in the wire."
}

72. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Optical switching of electron transport in a waveguide-QED system.
Physica E: Low-dimensional Systems and Nanostructures 84, 280 - 284 (2016).
Abstract Abstract Electron switching in waveguides coupled to a photon cavity is found to be strongly influenced by the photon energy and polarization. Therefore, the charge dynamics in the system is investigated in two different regimes, for off-resonant and resonant photon fields. In the off-resonant photon field, the photon energy is smaller than the energy spacing between the first two lowest subbands of the waveguide system, the charge splits between the waveguides implementing a NOT -quantum logic gate action. In the resonant photon field, the charge is totally switched from one waveguide to the other due to the appearance of photon replica states of the first subband in the second subband region instigating a quantum-NOT transition. In addition, the importance of the photon polarization to control the charge motion in the waveguide system is demonstrated. The idea of charge switching in electronic circuits may serve to built quantum bits.
URL arXiv, DOI BibTeX

@article{Abdullah2016280,
title = "Optical switching of electron transport in a waveguide-QED system",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 84,
number = "",
pages = "280 - 284",
year = 2016,
note = "",
issn = "1386-9477",
doi = "http://dx.doi.org/10.1016/j.physe.2016.06.023",
url = "http://www.sciencedirect.com/science/article/pii/S1386947716306749",
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
arxiv = "http://arxiv.org/abs/1602.04979",
keywords = "Quantum interference devices",
abstract = "Abstract Electron switching in waveguides coupled to a photon cavity is found to be strongly influenced by the photon energy and polarization. Therefore, the charge dynamics in the system is investigated in two different regimes, for off-resonant and resonant photon fields. In the off-resonant photon field, the photon energy is smaller than the energy spacing between the first two lowest subbands of the waveguide system, the charge splits between the waveguides implementing a \{NOT\} -quantum logic gate action. In the resonant photon field, the charge is totally switched from one waveguide to the other due to the appearance of photon replica states of the first subband in the second subband region instigating a quantum-NOT transition. In addition, the importance of the photon polarization to control the charge motion in the waveguide system is demonstrated. The idea of charge switching in electronic circuits may serve to built quantum bits."
}

73. Razvan Chirla, Andrei Manolescu and Cătălin Paşcu Moca Moca.
Transmission of a microwave cavity coupled to localized Shiba states.
Phys. Rev. B 93, 155110 (April 2016).
Abstract We consider a strongly correlated quantum dot, tunnel coupled to two superconducting leads and capacitively coupled to a single mode microwave cavity. When the superconducting gap is the largest energy scale, multiple Shiba states are formed inside the gap. The competition of these states for the ground state signals a quantum phase transition. We demonstrate that photonic measurements can be used to probe such localized Shiba states. Moreover, the quantum phase transition can be pinpointed exactly from the sudden change in the transmission signal. Calculations were performed using the numerical renormalization-group approach.
URL arXiv, DOI BibTeX

@article{PhysRevB.93.155110,
title = "Transmission of a microwave cavity coupled to localized Shiba states",
author = "Chirla, Razvan and Manolescu, Andrei and Moca, Cătălin Paşcu Moca",
journal = "Phys. Rev. B",
volume = 93,
issue = 15,
pages = 155110,
numpages = 9,
year = 2016,
month = "Apr",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.93.155110",
arxiv = "http://arxiv.org/abs/1512.08093",
abstract = "We consider a strongly correlated quantum dot, tunnel coupled to two superconducting leads and capacitively coupled to a single mode microwave cavity. When the superconducting gap is the largest energy scale, multiple Shiba states are formed inside the gap. The competition of these states for the ground state signals a quantum phase transition. We demonstrate that photonic measurements can be used to probe such localized Shiba states. Moreover, the quantum phase transition can be pinpointed exactly from the sudden change in the transmission signal. Calculations were performed using the numerical renormalization-group approach."
}

74. Anna Sitek, Gunnar Thorgilsson, Vidar Gudmundsson and Andrei Manolescu.
Multi-domain electromagnetic absorption of triangular quantum rings.
Nanotechnology 27, 225202 (2016).
Abstract We present a theoretical study of the unielectronic energy spectra, electron localization, and optical absorption of triangular core–shell quantum rings. We show how these properties depend on geometric details of the triangle, such as side thickness or corners’ symmetry. For equilateral triangles, the lowest six energy states (including spin) are grouped in an energy shell, are localized only around corner areas, and are separated by a large energy gap from the states with higher energy which are localized on the sides of the triangle. The energy levels strongly depend on the aspect ratio of the triangle sides, i.e., thickness/length ratio, in such a way that the energy differences are not monotonous functions of this ratio. In particular, the energy gap between the group of states localized in corners and the states localized on the sides strongly decreases with increasing the side thickness, and then slightly increases for thicker samples. With increasing the thickness the low-energy shell remains distinct but the spatial distribution of these states spreads. The behavior of the energy levels and localization leads to a thickness-dependent absorption spectrum where one transition may be tuned in the THz domain and a second transition can be tuned from THz to the infrared range of electromagnetic spectrum. We show how these features may be further controlled with an external magnetic field. In this work the electron–electron Coulomb repulsion is neglected.
URL arXiv, DOI BibTeX

@article{0957-4484-27-22-225202,
author = "Anna Sitek and Gunnar Thorgilsson and Vidar Gudmundsson and Andrei Manolescu",
title = "Multi-domain electromagnetic absorption of triangular quantum rings",
journal = "Nanotechnology",
volume = 27,
number = 22,
pages = 225202,
url = "http://stacks.iop.org/0957-4484/27/i=22/a=225202",
year = 2016,
doi = "10.1088/0957-4484/27/22/225202",
arxiv = "http://arxiv.org/abs/1511.05596",
abstract = "We present a theoretical study of the unielectronic energy spectra, electron localization, and optical absorption of triangular core–shell quantum rings. We show how these properties depend on geometric details of the triangle, such as side thickness or corners’ symmetry. For equilateral triangles, the lowest six energy states (including spin) are grouped in an energy shell, are localized only around corner areas, and are separated by a large energy gap from the states with higher energy which are localized on the sides of the triangle. The energy levels strongly depend on the aspect ratio of the triangle sides, i.e., thickness/length ratio, in such a way that the energy differences are not monotonous functions of this ratio. In particular, the energy gap between the group of states localized in corners and the states localized on the sides strongly decreases with increasing the side thickness, and then slightly increases for thicker samples. With increasing the thickness the low-energy shell remains distinct but the spatial distribution of these states spreads. The behavior of the energy levels and localization leads to a thickness-dependent absorption spectrum where one transition may be tuned in the THz domain and a second transition can be tuned from THz to the infrared range of electromagnetic spectrum. We show how these features may be further controlled with an external magnetic field. In this work the electron–electron Coulomb repulsion is neglected."
}

75. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Cavity-Photon Controlled Thermoelectric Transport through a Quantum Wire.
ACS Photonics 3, 249-254 (2016).
Abstract We investigate the influence of a quantized photon field on thermoelectric transport of electrons through a quantum wire embedded in a photon cavity. The quantum wire is connected to two electron reservoirs at different temperatures leading to the generation of a thermoelectric current. The transient thermoelectric current strongly depends on the photon energy and the number of photons initially in the cavity. Two different regimes are studied, off-resonant and on-resonant polarized fields, with photon energy smaller than, or equal to, the energy spacing between some of the lowest states in the quantum wire. We observe that the current is inverted for the off-resonant photon field due to participation of photon replica states in the transport. A reduction in the current is recorded for the resonant photon field, a direct consequence of the Rabi-splitting.
URL, DOI BibTeX

@article{doi:10.1021/acsphotonics.5b00532,
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Cavity-Photon Controlled Thermoelectric Transport through a Quantum Wire",
journal = "ACS Photonics",
volume = 3,
number = 2,
pages = "249-254",
year = 2016,
doi = "10.1021/acsphotonics.5b00532",
url = "http://dx.doi.org/10.1021/acsphotonics.5b00532",
eprint = "http://dx.doi.org/10.1021/acsphotonics.5b00532",
abstract = "We investigate the influence of a quantized photon field on thermoelectric transport of electrons through a quantum wire embedded in a photon cavity. The quantum wire is connected to two electron reservoirs at different temperatures leading to the generation of a thermoelectric current. The transient thermoelectric current strongly depends on the photon energy and the number of photons initially in the cavity. Two different regimes are studied, off-resonant and on-resonant polarized fields, with photon energy smaller than, or equal to, the energy spacing between some of the lowest states in the quantum wire. We observe that the current is inverted for the off-resonant photon field due to participation of photon replica states in the transport. A reduction in the current is recorded for the resonant photon field, a direct consequence of the Rabi-splitting."
}

76. Vidar Gudmundsson, Anna Sitek, Nzar Rauf Abdullah, Chi-Shung Tang and Andrei Manolescu.
Cavity-photon contribution to the effective interaction of electrons in parallel quantum dots.
Annalen der Physik 528, 394–403 (2016).
Abstract A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field.
URL arXiv, DOI BibTeX

@article{ANDP:ANDP201500298,
author = "Gudmundsson, Vidar and Sitek, Anna and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei",
title = "Cavity-photon contribution to the effective interaction of electrons in parallel quantum dots",
journal = "Annalen der Physik",
volume = 528,
number = 5,
issn = "1521-3889",
url = "http://dx.doi.org/10.1002/andp.201500298",
doi = "10.1002/andp.201500298",
arxiv = "http://arxiv.org/abs/1505.03181",
pages = "394--403",
year = 2016,
abstract = "A single cavity photon mode is expected to modify the Coulomb interaction of an electron system in the cavity. Here we investigate this phenomena in a parallel double quantum dot system. We explore properties of the closed system and the system after it has been opened up for electron transport. We show how results for both cases support the idea that the effective electron-electron interaction becomes more repulsive in the presence of a cavity photon field. This can be understood in terms of the cavity photons dressing the polarization terms in the effective mutual electron interaction leading to nontrivial delocalization or polarization of the charge in the double parallel dot potential. In addition, we find that the effective repulsion of the electrons can be reduced by quadrupolar collective oscillations excited by an external classical dipole electric field."
}

77. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot.
Journal of Physics: Condensed Matter 28, 375301 (2016).
Abstract We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, {$\hslash {ømega_\gamma}/\hslash {ømega_c}$} , between the photon energy, {$\hslash {ømega_\gamma}$} , and the cyclotron energy,{$\hslash {ømega_c}$}. Enhancement in the electron transport with increasing electron–photon coupling is observed when {$\hslash {ømega_\gamma}/\hslash {ømega_c}>1$}. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contact area for the leads. Suppression in the electron transport is found when {$\hslash {ømega_\gamma}/\hslash {ømega_c}<1$}, as the external magnetic field causes circular confinement of the charge density around the dot.
URL arXiv, DOI BibTeX

@article{0953-8984-28-37-375301,
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot",
journal = "Journal of Physics: Condensed Matter",
volume = 28,
number = 37,
pages = 375301,
url = "http://stacks.iop.org/0953-8984/28/i=37/a=375301",
arxiv = "http://arxiv.org/abs/1512.00392",
doi = "10.1088/0953-8984/28/37/375301",
year = 2016,
abstract = "We investigate theoretically the balance of the static magnetic and the dynamical photon forces in the electron transport through a quantum dot in a photon cavity with a single photon mode. The quantum dot system is connected to external leads and the total system is exposed to a static perpendicular magnetic field. We explore the transport characteristics through the system by tuning the ratio, {$\hslash {{\omega}_{\gamma}}/\hslash {{\omega}_{c}}$} , between the photon energy, {$\hslash {{\omega}_{\gamma}}$} , and the cyclotron energy,{$\hslash {{\omega}_{c}}$}. Enhancement in the electron transport with increasing electron–photon coupling is observed when {$\hslash {{\omega}_{\gamma}}/\hslash {{\omega}_{c}}>1$}. In this case the photon field dominates and stretches the electron charge distribution in the quantum dot, extending it towards the contact area for the leads. Suppression in the electron transport is found when {$\hslash {{\omega}_{\gamma}}/\hslash {{\omega}_{c}}<1$}, as the external magnetic field causes circular confinement of the charge density around the dot."
}

78. Marjan Ilkov, Kristinn Torfason, Andrei Manolescu and Ágúst Valfells.
Terahertz pulsed photogenerated current in microdiodes at room temperature.
Applied Physics Letters 107, (2015).
Abstract Space-charge modulation of the current in a vacuum diode under photoemission leads to the formation of beamlets with time periodicity corresponding to THz frequencies. We investigate the effect of the emitter temperature and internal space-charge forces on the formation and persistence of the beamlets. We find that temperature effects are most important for beam degradation at low values of the applied electric field, whereas at higher fields, intra-beamlet space-charge forces are dominant. The current modulation is most robust when there is only one beamlet present in the diode gap at a time, corresponding to a macroscopic version of the Coulomb blockade. It is shown that a vacuum microdiode can operate quite well as a tunable THz oscillator at room temperature with an applied electric field above 10 MV/m and a diode gap of the order of 100 nm.
URL arXiv, DOI BibTeX

@article{ilkov2015terahertz,
author = "Ilkov, Marjan and Torfason, Kristinn and Manolescu, Andrei and Valfells, Ágúst",
title = "Terahertz pulsed photogenerated current in microdiodes at room temperature",
journal = "Applied Physics Letters",
year = 2015,
volume = 107,
number = 20,
eid = 203508,
pages = "",
url = "http://scitation.aip.org/content/aip/journal/apl/107/20/10.1063/1.4936176",
doi = "10.1063/1.4936176",
abstract = "Space-charge modulation of the current in a vacuum diode under photoemission leads to the formation of beamlets with time periodicity corresponding to THz frequencies. We investigate the effect of the emitter temperature and internal space-charge forces on the formation and persistence of the beamlets. We find that temperature effects are most important for beam degradation at low values of the applied electric field, whereas at higher fields, intra-beamlet space-charge forces are dominant. The current modulation is most robust when there is only one beamlet present in the diode gap at a time, corresponding to a macroscopic version of the Coulomb blockade. It is shown that a vacuum microdiode can operate quite well as a tunable THz oscillator at room temperature with an applied electric field above 10 MV/m and a diode gap of the order of 100 nm.",
arxiv = "http://arxiv.org/abs/1508.06308"
}

79. Błażej Jaworowski, Andrei Manolescu and Paweł Potasz.
Fractional Chern insulator phase at the transition between checkerboard and Lieb lattices.
Phys. Rev. B 92, 245119 (December 2015).
Abstract The stability of the ν=1/3 fractional Chern insulator (FCI) phase is analyzed on the example of a checkerboard lattice undergoing a transition into a Lieb lattice. The transition is performed by the addition of a second sublattice, whose coupling to the checkerboard sites is controlled by sublattice staggered potential. We investigate the influence of these sites on the many-body energy gap between three lowest energy states and the fourth state. We consider cases with different complex phases acquired in hopping and a model with a flattened topologically nontrivial band. We find that an interaction with the additional sites either open the single-particle gap or enlarge the existing one, which translates into similar effect on the many-particle gap. By looking at Berry curvature flatness we notice its strong correlation with the magnitude of the many-body energy gap, suggesting that the main mechanism of the FCI stabilization by additional atoms is via their influence on the Berry curvature. Evidence of the FCI phase for a region in a parameter space with larger energy gap is shown by looking at momenta of the threefold degenerate ground state, spectral flow, and quasihole excitation spectrum.
URL arXiv, DOI BibTeX

@article{PhysRevB.92.245119,
title = "Fractional Chern insulator phase at the transition between checkerboard and Lieb lattices",
author = "Błażej Jaworowski and Manolescu, Andrei and Potasz, Paweł",
journal = "Phys. Rev. B",
volume = 92,
issue = 24,
pages = 245119,
numpages = 7,
year = 2015,
month = "Dec",
publisher = "American Physical Society",
arxiv = "http://arxiv.org/abs/1508.04399",
doi = "10.1103/PhysRevB.92.245119",
abstract = "The stability of the ν=1/3 fractional Chern insulator (FCI) phase is analyzed on the example of a checkerboard lattice undergoing a transition into a Lieb lattice. The transition is performed by the addition of a second sublattice, whose coupling to the checkerboard sites is controlled by sublattice staggered potential. We investigate the influence of these sites on the many-body energy gap between three lowest energy states and the fourth state. We consider cases with different complex phases acquired in hopping and a model with a flattened topologically nontrivial band. We find that an interaction with the additional sites either open the single-particle gap or enlarge the existing one, which translates into similar effect on the many-particle gap. By looking at Berry curvature flatness we notice its strong correlation with the magnitude of the many-body energy gap, suggesting that the main mechanism of the FCI stabilization by additional atoms is via their influence on the Berry curvature. Evidence of the FCI phase for a region in a parameter space with larger energy gap is shown by looking at momenta of the threefold degenerate ground state, spectral flow, and quasihole excitation spectrum."
}

80. Charles Goehry, George Alexandru Nemnes and Andrei Manolescu.
Collective Behavior of Molecular Dipoles in CH$_3$NH$_3$PbI$_3$.
The Journal of Physical Chemistry C 119, 19674-19680 (2015).
Abstract Using ab-initio molecular dynamics, we report a detailed exploration of the thermal motion occurring in perovskite crystals of formula CH$_3$NH$_3$PbI$_3$. We exploit the data generated to obtain estimates of the rotational relaxation time of the cation (CH$_3$NH$_3$+). We examine the tetragonal and cubic phase, as both may be present under operational conditions. Influenced by each other, and by the tilting of PBI$_6$ octahedra, cations undergo collective motion as their contribution to polarization does not vanish. We thereby qualitatively describe the modus operandi of formation of microscopic ferroelectric domains.
URL, DOI BibTeX

@article{jpcc-5b05823,
author = "Goehry, Charles and Nemnes, George Alexandru and Manolescu, Andrei",
title = "Collective Behavior of Molecular Dipoles in CH$_3$NH$_3$PbI$_3$",
journal = "The Journal of Physical Chemistry C",
volume = 119,
number = 34,
pages = "19674-19680",
year = 2015,
doi = "10.1021/acs.jpcc.5b05823",
url = "http://dx.doi.org/10.1021/acs.jpcc.5b05823",
eprint = "http://dx.doi.org/10.1021/acs.jpcc.5b05823",
abstract = "Using ab-initio molecular dynamics, we report a detailed exploration of the thermal motion occurring in perovskite crystals of formula CH$_3$NH$_3$PbI$_3$. We exploit the data generated to obtain estimates of the rotational relaxation time of the cation (CH$_3$NH$_3$+). We examine the tetragonal and cubic phase, as both may be present under operational conditions. Influenced by each other, and by the tilting of PBI$_6$ octahedra, cations undergo collective motion as their contribution to polarization does not vanish. We thereby qualitatively describe the modus operandi of formation of microscopic ferroelectric domains."
}

81. A Sitek, V Gudmundsson and A Manolescu.
Symmetry dependent electron localization and optical absorption of polygonal quantum rings.
In Transparent Optical Networks (ICTON), 2015 17th International Conference on. (July 2015), 1-4.
Abstract We compare energy spectra, electron localization and optical absorption of square and diamond quantum rings and analyze how sample geometry affects those features. We show that low energy levels of diamond rings form two groups delocalized between opposite corners which results in increased number of optical transitions. We also show that contacts applied to corner areas allow for continuous change between square- and diamond-like behavior of the same sample, irrespective of its shape.
arXiv, DOI BibTeX

@inproceedings{7193541,
author = "Sitek, A. and Gudmundsson, V. and Manolescu, A.",
booktitle = "Transparent Optical Networks (ICTON), 2015 17th International Conference on",
title = "Symmetry dependent electron localization and optical absorption of polygonal quantum rings",
year = 2015,
pages = "1-4",
abstract = "We compare energy spectra, electron localization and optical absorption of square and diamond quantum rings and analyze how sample geometry affects those features. We show that low energy levels of diamond rings form two groups delocalized between opposite corners which results in increased number of optical transitions. We also show that contacts applied to corner areas allow for continuous change between square- and diamond-like behavior of the same sample, irrespective of its shape.",
keywords = "light absorption;quantum wires;optical transition;polygonal quantum rings optical absorption;symmetry dependent electron localization;Absorption;Diamonds;Electron optics;Energy states;Integrated optics;Nanowires;Probability distribution;absorption;core-multi-shell structures;polygonal quantum rings",
doi = "10.1109/ICTON.2015.7193541",
arxiv = "http://arxiv.org/abs/1505.00207",
month = "July"
}

82. Sigurdur I Erlingsson, Andrei Manolescu and D C Marinescu.
Asymmetric Landau bands due to spin–orbit coupling.
Journal of Physics: Condensed Matter 27, 225303 (2015).
Abstract We show that the Landau bands obtained in a two-dimensional lateral semiconductor superlattice with spin–orbit coupling (SOC) of the Rashba/Dresselhaus type, linear in the electron momentum, placed in a tilted magnetic field, do not follow the symmetry of the spatial modulation. Moreover, this phenomenology is found to depend on the relative tilt of magnetic field and on the SOC type: (a) when only Rashba SOC exists and the magnetic field is tilted in the direction of the superlattice (b) Dresselhaus SOC exists and the magnetic field is tilted in the direction perpendicular to the superlattice. Consequently, measurable properties of the modulated system become anisotropic in a tilted magnetic field when the field is conically rotated around the z axis, at a fixed polar angle, as we demonstrate by calculating the resistivity and the magnetization.
URL arXiv, DOI BibTeX

@article{0953-8984-27-22-225303,
author = "Sigurdur I Erlingsson and Andrei Manolescu and D C Marinescu",
title = "Asymmetric Landau bands due to spin–orbit coupling",
journal = "Journal of Physics: Condensed Matter",
volume = 27,
number = 22,
pages = 225303,
url = "http://stacks.iop.org/0953-8984/27/i=22/a=225303",
year = 2015,
doi = "10.1088/0953-8984/27/22/225303",
abstract = "We show that the Landau bands obtained in a two-dimensional lateral semiconductor superlattice with spin–orbit coupling (SOC) of the Rashba/Dresselhaus type, linear in the electron momentum, placed in a tilted magnetic field, do not follow the symmetry of the spatial modulation. Moreover, this phenomenology is found to depend on the relative tilt of magnetic field and on the SOC type: (a) when only Rashba SOC exists and the magnetic field is tilted in the direction of the superlattice (b) Dresselhaus SOC exists and the magnetic field is tilted in the direction perpendicular to the superlattice. Consequently, measurable properties of the modulated system become anisotropic in a tilted magnetic field when the field is conically rotated around the z axis, at a fixed polar angle, as we demonstrate by calculating the resistivity and the magnetization.",
arxiv = "http://arxiv.org/abs/1504.04699"
}

83. Jeremy Capps, D C Marinescu and Andrei Manolescu.
Coulomb interaction effects in a two-dimensional quantum well with spin-orbit interaction.
Phys. Rev. B 91, 165301 (April 2015).
Abstract Starting from general total-energy considerations, we demonstrate that the SU(2) spin-rotation symmetry and the resulting persistent helical state (PHS) predicted to occur in an electron system with equal Rashba-Dresselhaus coupling constants are not in fact realized. On account of the accidental degeneracy that appears in the single-particle spectrum, the Coulomb interaction favors the apparition of an itinerant antiferromagnetic (IAF) order characterized by a fractional polarization of fixed spatial orientation. Within the Hartree-Fock approximation, we obtain numerical results that describe the temperature evolution of the order parameter in the IAF state and determine the critical temperature of the transition to the paramagnetic state.
URL, DOI BibTeX

@article{PhysRevB.91.165301,
title = "Coulomb interaction effects in a two-dimensional quantum well with spin-orbit interaction",
author = "Capps, Jeremy and Marinescu, D. C. and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 91,
issue = 16,
pages = 165301,
numpages = 8,
year = 2015,
month = "Apr",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.91.165301",
abstract = "Starting from general total-energy considerations, we demonstrate that the SU(2) spin-rotation symmetry and the resulting persistent helical state (PHS) predicted to occur in an electron system with equal Rashba-Dresselhaus coupling constants are not in fact realized. On account of the accidental degeneracy that appears in the single-particle spectrum, the Coulomb interaction favors the apparition of an itinerant antiferromagnetic (IAF) order characterized by a fractional polarization of fixed spatial orientation. Within the Hartree-Fock approximation, we obtain numerical results that describe the temperature evolution of the order parameter in the IAF state and determine the critical temperature of the transition to the paramagnetic state."
}

84. Anna Sitek, Llorenç Serra, Vidar Gudmundsson and Andrei Manolescu.
Electron localization and optical absorption of polygonal quantum rings.
Phys. Rev. B 91, 235429 (June 2015).
Abstract We investigate theoretically polygonal quantum rings and focus mostly on the triangular geometry where the corner effects are maximal. Such rings can be seen as short core-shell nanowires, a generation of semiconductor heterostructures with multiple applications. We show how the geometry of the sample determines the electronic energy spectrum, and also the localization of electrons, with effects on the optical absorption. In particular, we show that irrespective of the ring shape low-energy electrons are always attracted by corners and are localized in their vicinity. The absorption spectrum in the presence of a magnetic field shows only two peaks within the corner-localized state domain, each associated with different circular polarization. This picture may be changed by an external electric field which allows previously forbidden transitions, and thus enables the number of corners to be determined. We show that polygonal quantum rings allow absorption of waves from distant ranges of the electromagnetic spectrum within one sample.
URL arXiv, DOI BibTeX

@article{PhysRevB.91.235429,
title = "Electron localization and optical absorption of polygonal quantum rings",
author = "Sitek, Anna and Serra, Lloren\c{c} and Gudmundsson, Vidar and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 91,
issue = 23,
pages = 235429,
numpages = 10,
year = 2015,
month = "Jun",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.91.235429",
arxiv = "http://arxiv.org/abs/1503.09186",
abstract = "We investigate theoretically polygonal quantum rings and focus mostly on the triangular geometry where the corner effects are maximal. Such rings can be seen as short core-shell nanowires, a generation of semiconductor heterostructures with multiple applications. We show how the geometry of the sample determines the electronic energy spectrum, and also the localization of electrons, with effects on the optical absorption. In particular, we show that irrespective of the ring shape low-energy electrons are always attracted by corners and are localized in their vicinity. The absorption spectrum in the presence of a magnetic field shows only two peaks within the corner-localized state domain, each associated with different circular polarization. This picture may be changed by an external electric field which allows previously forbidden transitions, and thus enables the number of corners to be determined. We show that polygonal quantum rings allow absorption of waves from distant ranges of the electromagnetic spectrum within one sample."
}

85. Vidar Gudmundsson, Anna Sitek, Pei-yi Lin, Nzar Rauf Abdullah, Chi-Shung Tang and Andrei Manolescu.
Coupled Collective and Rabi Oscillations Triggered by Electron Transport through a Photon Cavity.
ACS Photonics 2, 930-934 (2015).
Abstract We show how the switching on of electron transport through a system of two parallel quantum dots embedded in a short quantum wire in a photon cavity can trigger coupled Rabi and collective electron–photon oscillations. We select the initial state of the system to be an eigenstate of the closed system containing two Coulomb-interacting electrons with possibly few photons of a single cavity mode. The many-level quantum dots are described by a continuous potential. The Coulomb interaction and the para- and diamagnetic electron–photon interactions are treated by exact diagonalization in a truncated Fock space. To identify the collective modes, the results are compared for an open and a closed system with respect to the coupling to external electron reservoirs, or leads. We demonstrate that the vacuum Rabi oscillations can be seen in transport quantities as the current in and out of the system.
URL arXiv, DOI BibTeX

@article{doi:10.1021/acsphotonics.5b00115,
author = "Gudmundsson, Vidar and Sitek, Anna and Lin, Pei-yi and Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei",
title = "Coupled Collective and Rabi Oscillations Triggered by Electron Transport through a Photon Cavity",
journal = "ACS Photonics",
volume = 2,
number = 7,
pages = "930-934",
year = 2015,
doi = "10.1021/acsphotonics.5b00115",
arxiv = "http://arxiv.org/abs/1502.06242",
url = "http://dx.doi.org/10.1021/acsphotonics.5b00115",
eprint = "http://dx.doi.org/10.1021/acsphotonics.5b00115",
abstract = "We show how the switching on of electron transport through a system of two parallel quantum dots embedded in a short quantum wire in a photon cavity can trigger coupled Rabi and collective electron–photon oscillations. We select the initial state of the system to be an eigenstate of the closed system containing two Coulomb-interacting electrons with possibly few photons of a single cavity mode. The many-level quantum dots are described by a continuous potential. The Coulomb interaction and the para- and diamagnetic electron–photon interactions are treated by exact diagonalization in a truncated Fock space. To identify the collective modes, the results are compared for an open and a closed system with respect to the coupling to external electron reservoirs, or leads. We demonstrate that the vacuum Rabi oscillations can be seen in transport quantities as the current in and out of the system."
}

86. Kristinn Torfason, Agust Valfells and Andrei Manolescu.
Molecular dynamics simulations of field emission from a planar nanodiode.
Physics of Plasmas (1994-present) 22, - (2015).
Abstract High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission in planar nanodiodes. The effects of space-charge and emitter radius are examined and compared to previous results concerning transition from Fowler-Nordheim to Child-Langmuir current [Y. Y. Lau, Y. Liu, and R. K. Parker, Phys. Plasmas 1, 2082 (1994) and Y. Feng and J. P. Verboncoeur, Phys. Plasmas 13, 073105 (2006)]. The Fowler-Nordheim law is used to determine the current density injected into the system and the Metropolis-Hastings algorithm to find a favourable point of emission on the emitter surface. A simple fluid like model is also developed and its results are in qualitative agreement with the simulations.
URL arXiv, DOI BibTeX

@article{4914855,
author = "Torfason, Kristinn and Valfells, Agust and Manolescu, Andrei",
title = "Molecular dynamics simulations of field emission from a planar nanodiode",
journal = "Physics of Plasmas (1994-present)",
year = 2015,
volume = 22,
number = 3,
eid = 033109,
pages = "-",
url = "http://scitation.aip.org/content/aip/journal/pop/22/3/10.1063/1.4914855",
doi = "http://dx.doi.org/10.1063/1.4914855",
arxiv = "http://arxiv.org/abs/1412.4537",
abstract = "High resolution molecular dynamics simulations with full Coulomb interactions of electrons are used to investigate field emission in planar nanodiodes. The effects of space-charge and emitter radius are examined and compared to previous results concerning transition from Fowler-Nordheim to Child-Langmuir current [Y. Y. Lau, Y. Liu, and R. K. Parker, Phys. Plasmas 1, 2082 (1994) and Y. Feng and J. P. Verboncoeur, Phys. Plasmas 13, 073105 (2006)]. The Fowler-Nordheim law is used to determine the current density injected into the system and the Metropolis-Hastings algorithm to find a favourable point of emission on the emitter surface. A simple fluid like model is also developed and its results are in qualitative agreement with the simulations."
}

87. Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Excitation spectra of a quantum ring embedded in a photon cavity.
Journal of Optics 17, 015201 (2015).
Abstract We explore the response of a quantum ring system coupled to a photon cavity with a single mode when excited by a classical dipole field. We find that the energy oscillates between the electronic and photonic components of the system. The contribution of the linear and the quadratic terms in the vector potential to the electron–photon interaction energy are of similar magnitude, but opposite signs stressing the importance of retaining both in the model. Furthermore, we find different Fourier spectra for the oscillations of the center of charge and the oscillations of the mean photon number in time. The Fourier spectra are compared to the spectrum of the many-body (MB) states and selection rules discussed. In case of the center of charge oscillations, the dipole matrix elements preselect the allowed Bohr frequencies of the transitions, while for the oscillations of the mean photon number, the difference of the photon content of the MB states influences the selection rules.
URL arXiv, DOI BibTeX

@article{2040-8986-17-1-015201,
author = "Thorsten Arnold and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Excitation spectra of a quantum ring embedded in a photon cavity",
journal = "Journal of Optics",
volume = 17,
number = 1,
pages = 015201,
url = "http://stacks.iop.org/2040-8986/17/i=1/a=015201",
year = 2015,
arxiv = "http://arxiv.org/abs/1410.0174",
doi = "10.1088/2040-8978/17/1/015201",
abstract = "We explore the response of a quantum ring system coupled to a photon cavity with a single mode when excited by a classical dipole field. We find that the energy oscillates between the electronic and photonic components of the system. The contribution of the linear and the quadratic terms in the vector potential to the electron–photon interaction energy are of similar magnitude, but opposite signs stressing the importance of retaining both in the model. Furthermore, we find different Fourier spectra for the oscillations of the center of charge and the oscillations of the mean photon number in time. The Fourier spectra are compared to the spectrum of the many-body (MB) states and selection rules discussed. In case of the center of charge oscillations, the dipole matrix elements preselect the allowed Bohr frequencies of the transitions, while for the oscillations of the mean photon number, the difference of the photon content of the MB states influences the selection rules."
}

88. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Coherent transient transport of interacting electrons through a quantum waveguide switch.
Journal of Physics: Condensed Matter 27, 015301 (2015).
Abstract We investigate coherent electron-switching transport in a double quantum waveguide system in a perpendicular static or vanishing magnetic field. The finite symmetric double waveguide is connected to two semi-infinite leads from both ends. The double waveguide can be defined as two parallel finite quantum wires or waveguides coupled via a window to facilitate coherent electron inter-wire transport. By tuning the length of the coupling window, we observe oscillations in the net charge current and a maximum electron conductance for the energy levels of the two waveguides in resonance. The importance of the mutual Coulomb interaction between the electrons and the influence of two-electron states is clarified by comparing results with and without the interaction. Even though the Coulomb interaction can lift two-electron states out of the group of active transport states the length of the coupling window can be tuned to locate two very distinct transport modes in the system in the late transient regime before the onset of a steady state. A static external magnetic field and quantum-dots formed by side gates (side quantum dots) can be used to enhance the inter-waveguide transport which can serve to implement a quantum logic device. The fact that the device can be operated in the transient regime can be used to enhance its speed.
URL arXiv, DOI BibTeX

@article{0953-8984-27-1-015301,
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
title = "Coherent transient transport of interacting electrons through a quantum waveguide switch",
journal = "Journal of Physics: Condensed Matter",
volume = 27,
number = 1,
pages = 015301,
url = "http://stacks.iop.org/0953-8984/27/i=1/a=015301",
year = 2015,
arxiv = "http://arxiv.org/abs/1408.1007",
doi = "10.1088/0953-8984/27/1/015301",
abstract = "We investigate coherent electron-switching transport in a double quantum waveguide system in a perpendicular static or vanishing magnetic field. The finite symmetric double waveguide is connected to two semi-infinite leads from both ends. The double waveguide can be defined as two parallel finite quantum wires or waveguides coupled via a window to facilitate coherent electron inter-wire transport. By tuning the length of the coupling window, we observe oscillations in the net charge current and a maximum electron conductance for the energy levels of the two waveguides in resonance. The importance of the mutual Coulomb interaction between the electrons and the influence of two-electron states is clarified by comparing results with and without the interaction. Even though the Coulomb interaction can lift two-electron states out of the group of active transport states the length of the coupling window can be tuned to locate two very distinct transport modes in the system in the late transient regime before the onset of a steady state. A static external magnetic field and quantum-dots formed by side gates (side quantum dots) can be used to enhance the inter-waveguide transport which can serve to implement a quantum logic device. The fact that the device can be operated in the transient regime can be used to enhance its speed."
}

89. M Ilkov, K Torfason, A Manolescu and A Valfells.
Synchronization in Arrays of Vacuum Microdiodes.
Electron Devices, IEEE Transactions on PP, 1-1 (2014).
Abstract Simulations have shown that space-charge effects can lead to regular modulation of photoemitted beams in vacuum diodes with gap sizes on the order of 1 μm and accelerating voltage on the order of $1$ V. These modulations are in the terahertz regime and can be tuned by simply changing the emitter area or accelerating vacuum field. The average current in the diode corresponds to the Child–Langmuir current, but the amplitude of the oscillations is affected by various factors. Given the small size and voltage of the system, the maximum radiated ac power is expected to be small. In this paper, we show that an array of small emitters produces higher frequency signals than a single large emitter of the same area and how these emitters may be synchronized to produce higher power signals.
arXiv, DOI BibTeX

@article{6979259,
author = "Ilkov, M. and Torfason, K. and Manolescu, A. and Valfells, A.",
journal = "Electron Devices, IEEE Transactions on",
title = "Synchronization in Arrays of Vacuum Microdiodes",
year = 2014,
month = "",
volume = "PP",
number = 99,
pages = "1-1",
abstract = "Simulations have shown that space-charge effects can lead to regular modulation of photoemitted beams in vacuum diodes with gap sizes on the order of 1 μm and accelerating voltage on the order of $1$ V. These modulations are in the terahertz regime and can be tuned by simply changing the emitter area or accelerating vacuum field. The average current in the diode corresponds to the Child--Langmuir current, but the amplitude of the oscillations is affected by various factors. Given the small size and voltage of the system, the maximum radiated ac power is expected to be small. In this paper, we show that an array of small emitters produces higher frequency signals than a single large emitter of the same area and how these emitters may be synchronized to produce higher power signals.",
keywords = "Cathodes;Couplings;Frequency synchronization;Oscillators;Quantum cascade lasers;Space charge;Synchronization;Synchronization;terahertz;vacuum microelectronics.",
doi = "10.1109/TED.2014.2370680",
issn = "0018-9383",
arxiv = "http://arxiv.org/abs/1409.0516"
}

90. Tomas Orn Rosdahl, Andrei Manolescu and Vidar Gudmundsson.
Signature of Snaking States in the Conductance of Core–Shell Nanowires.
Nano Letters (2014).
Abstract We model a core–shell nanowire (CSN) by a cylindrical surface of finite length. A uniform magnetic field perpendicular to the axis of the cylinder forms electron states along the lines of zero radial field projection, which can classically be described as snaking states. In a strong field, these states converge pairwise to quasidegenerate levels, which are situated at the bottom of the energy spectrum. We calculate the conductance of the CSN by coupling it to leads and predict that the snaking states govern transport at low chemical potential, forming isolated peaks, each of which may be split in two by applying a transverse electric field. If the contacts with the leads do not completely surround the CSN, as is usually the case in experiments, the amplitude of the snaking peaks changes when the magnetic field is rotated, determined by the overlap of the contacts with the snaking states.
URL arXiv, DOI BibTeX

@article{doi:10.1021/nl503499w,
author = "Rosdahl, Tomas Orn and Manolescu, Andrei and Gudmundsson, Vidar",
title = "Signature of Snaking States in the Conductance of Core–Shell Nanowires",
journal = "Nano Letters",
year = 2014,
doi = "10.1021/nl503499w",
note = "PMID: 25426964",
url = "http://dx.doi.org/10.1021/nl503499w",
arxiv = "http://arxiv.org/abs/1409.3429",
eprint = "http://dx.doi.org/10.1021/nl503499w",
abstract = "We model a core–shell nanowire (CSN) by a cylindrical surface of finite length. A uniform magnetic field perpendicular to the axis of the cylinder forms electron states along the lines of zero radial field projection, which can classically be described as snaking states. In a strong field, these states converge pairwise to quasidegenerate levels, which are situated at the bottom of the energy spectrum. We calculate the conductance of the CSN by coupling it to leads and predict that the snaking states govern transport at low chemical potential, forming isolated peaks, each of which may be split in two by applying a transverse electric field. If the contacts with the leads do not completely surround the CSN, as is usually the case in experiments, the amplitude of the snaking peaks changes when the magnetic field is rotated, determined by the overlap of the contacts with the snaking states."
}

91. Anda Elena Stanciu, George Alexandru Nemnes and Andrei Manolescu.
Thermoelectric effects in nanostructured quantum wires in the non-linear temperature regime.
arXiv preprint arXiv:1411.6018 (2014).
Abstract The thermoelectric voltage of a quantum dot connected to leads is calculated using the scattering R-matrix method. Our approach takes into account a temperature gradient between the contacts beyond the linear regime. We obtain sign changes of the thermopower when varying the temperature or the chemical potential around the resonances. The influence of the coupling strength of the contacts and of the thermoelectric field on the thermoelectric voltage is discussed.
arXiv BibTeX

@article{stanciu2014thermoelectric,
title = "Thermoelectric effects in nanostructured quantum wires in the non-linear temperature regime",
author = "Stanciu, Anda Elena and Nemnes, George Alexandru and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1411.6018",
arxiv = "http://arxiv.org/abs/1411.6018",
year = 2014,
abstract = "The thermoelectric voltage of a quantum dot connected to leads is calculated using the scattering R-matrix method. Our approach takes into account a temperature gradient between the contacts beyond the linear regime. We obtain sign changes of the thermopower when varying the temperature or the chemical potential around the resonances. The influence of the coupling strength of the contacts and of the thermoelectric field on the thermoelectric voltage is discussed."
}

92. Andreea A Nila, George Alexandru Nemnes and Andrei Manolescu.
Ab initio investigation of optical properties in triangular graphene-boron nitride core-shell nanostructures.
arXiv preprint arXiv:1411.6042 (2014).
Abstract We calculate the optical properties of atomic-sized core-shell graphene - boron nitride nanoflakes with triangular shaped crossection using the density functional theory. The optical properties can be tuned by using different sizes and proportions of the core-shell materials. Anisotropic effects manifested in the absorption of unpolarized light with different orientations of the optical vector are pointed out.
arXiv BibTeX

@article{nila2014ab,
title = "Ab initio investigation of optical properties in triangular graphene-boron nitride core-shell nanostructures",
author = "Nila, Andreea A and Nemnes, George Alexandru and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1411.6042",
arxiv = "http://arxiv.org/abs/1411.6042",
year = 2014,
abstract = "We calculate the optical properties of atomic-sized core-shell graphene - boron nitride nanoflakes with triangular shaped crossection using the density functional theory. The optical properties can be tuned by using different sizes and proportions of the core-shell materials. Anisotropic effects manifested in the absorption of unpolarized light with different orientations of the optical vector are pointed out."
}

93. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Cavity-photon-switched coherent transient transport in a double quantum waveguide.
Journal of Applied Physics 116, - (2014).
Abstract We study a cavity-photon-switched coherent electron transport in a symmetric double quantum waveguide. The waveguide system is weakly connected to two electron reservoirs, but strongly coupled to a single quantized photon cavity mode. A coupling window is placed between the waveguides to allow electron interference or inter-waveguide transport. The transient electron transport in the system is investigated using a quantum master equation. We present a cavity-photon tunable semiconductor quantum waveguide implementation of an inverter quantum gate, in which the output of the waveguide system may be selected via the selection of an appropriate photon number or “photon frequency” of the cavity. In addition, the importance of the photon polarization in the cavity, that is, either parallel or perpendicular to the direction of electron propagation in the waveguide system is demonstrated.
URL arXiv, DOI BibTeX

@article{abdullah2014cavity,
author = "Abdullah, Nzar Rauf and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
title = "Cavity-photon-switched coherent transient transport in a double quantum waveguide",
journal = "Journal of Applied Physics",
year = 2014,
volume = 116,
number = 23,
eid = 233104,
pages = "-",
url = "http://scitation.aip.org/content/aip/journal/jap/116/23/10.1063/1.4904907",
doi = "http://dx.doi.org/10.1063/1.4904907",
arxiv = "http://arxiv.org/abs/1410.4890",
abstract = "We study a cavity-photon-switched coherent electron transport in a symmetric double quantum waveguide. The waveguide system is weakly connected to two electron reservoirs, but strongly coupled to a single quantized photon cavity mode. A coupling window is placed between the waveguides to allow electron interference or inter-waveguide transport. The transient electron transport in the system is investigated using a quantum master equation. We present a cavity-photon tunable semiconductor quantum waveguide implementation of an inverter quantum gate, in which the output of the waveguide system may be selected via the selection of an appropriate photon number or “photon frequency” of the cavity. In addition, the importance of the photon polarization in the cavity, that is, either parallel or perpendicular to the direction of electron propagation in the waveguide system is demonstrated."
}

94. D C Marinescu, Andrei Manolescu and Jeremy Capps.
Anomalous spin and charge Seebeck effect in a quantum well with spin orbit interaction.
In Henri-Jean Drouhin, Jean-Eric Wegrowe and Manijeh Razeghi (eds.). Spintronics VII 9167. (2014), 207 – 216.
Abstract We discuss the possible existence of an anomalously high low-temperature charge and spin thermopower in a two dimensional electron system with Rashba and Dresselhaus spin-orbit coupling in the special case when the two interactions have equal strengths. The fundamental premise of the theory is the establishment of an weak itinerant antiferromagnetic order in the ground state, a spin alignment favored in the minimum-energy many-body state when the Coulomb interaction is considered. The transport in this state is modeled by using the solutions of a Boltzmann equation obtained within the relaxation time approximation. We show that when scattering on magnetic impurities is introduced, the energy dependence of the relaxation time enhances the value of the thermoelectric coefficient for both charge and spin currents. An estimate of the effect is provided for the case of a standard InAs quantum well and its variation with the strength of the magnetic scattering is studied.
URL, DOI BibTeX

@inproceedings{10.1117/12.2063671,
author = "D. C. Marinescu and Andrei Manolescu and Jeremy Capps",
title = "{Anomalous spin and charge Seebeck effect in a quantum well with spin orbit interaction}",
volume = 9167,
booktitle = "Spintronics VII",
editor = "Henri-Jean Drouhin and Jean-Eric Wegrowe and Manijeh Razeghi",
organization = "International Society for Optics and Photonics",
publisher = "SPIE",
pages = "207 -- 216",
abstract = "We discuss the possible existence of an anomalously high low-temperature charge and spin thermopower in a two dimensional electron system with Rashba and Dresselhaus spin-orbit coupling in the special case when the two interactions have equal strengths. The fundamental premise of the theory is the establishment of an weak itinerant antiferromagnetic order in the ground state, a spin alignment favored in the minimum-energy many-body state when the Coulomb interaction is considered. The transport in this state is modeled by using the solutions of a Boltzmann equation obtained within the relaxation time approximation. We show that when scattering on magnetic impurities is introduced, the energy dependence of the relaxation time enhances the value of the thermoelectric coefficient for both charge and spin currents. An estimate of the effect is provided for the case of a standard InAs quantum well and its variation with the strength of the magnetic scattering is studied.",
keywords = "spin-orbit, itinerant antiferromagnet, thermopower, spin Seebeck, Seebeck",
year = 2014,
doi = "10.1117/12.2063671",
url = "https://doi.org/10.1117/12.2063671"
}

95. Nzar Rauf Abdullah, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Delocalization of electrons by cavity photons in transport through a quantum dot molecule.
Physica E: Low-dimensional Systems and Nanostructures , - (2014).
Abstract Abstract We present results on cavity-photon-assisted electron transport through two lateral quantum dots embedded in a finite quantum wire. The double quantum dot system is weakly connected to two leads and strongly coupled to a single quantized photon cavity mode with initially two linearly polarized photons in the cavity. Including the full electron-photon interaction, the transient current controlled by a plunger-gate in the central system is studied by using quantum master equation. Without a photon cavity, two resonant current peaks are observed in the range selected for the plunger gate voltage: The ground state peak, and the peak corresponding to the first-excited state. The current in the ground state is higher than in the first-excited state due to their different symmetry. In a photon cavity with the photon field polarized along or perpendicular to the transport direction, two extra side peaks are found, namely, photon-replica of the ground state and photon-replica of the first-excited state. The side-peaks are caused by photon-assisted electron transport, with multiphoton absorption processes for up to three photons during an electron tunneling process. The inter-dot tunneling in the ground state can be controlled by the photon cavity in the case of the photon field polarized along the transport direction. The electron charge is delocalized from the dots by the photon cavity. Furthermore, the current in the photon-induced side-peaks can be strongly enhanced by increasing the electron-photon coupling strength for the case of photons polarized along the transport direction.
URL arXiv, DOI BibTeX

@article{RaufAbdullah2014,
title = "Delocalization of electrons by cavity photons in transport through a quantum dot molecule",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = "",
number = 0,
pages = "-",
year = 2014,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2014.07.030",
url = "http://www.sciencedirect.com/science/article/pii/S1386947714002938",
author = "Nzar Rauf Abdullah and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
keywords = "Electro-optical effects",
abstract = "Abstract We present results on cavity-photon-assisted electron transport through two lateral quantum dots embedded in a finite quantum wire. The double quantum dot system is weakly connected to two leads and strongly coupled to a single quantized photon cavity mode with initially two linearly polarized photons in the cavity. Including the full electron-photon interaction, the transient current controlled by a plunger-gate in the central system is studied by using quantum master equation. Without a photon cavity, two resonant current peaks are observed in the range selected for the plunger gate voltage: The ground state peak, and the peak corresponding to the first-excited state. The current in the ground state is higher than in the first-excited state due to their different symmetry. In a photon cavity with the photon field polarized along or perpendicular to the transport direction, two extra side peaks are found, namely, photon-replica of the ground state and photon-replica of the first-excited state. The side-peaks are caused by photon-assisted electron transport, with multiphoton absorption processes for up to three photons during an electron tunneling process. The inter-dot tunneling in the ground state can be controlled by the photon cavity in the case of the photon field polarized along the transport direction. The electron charge is delocalized from the dots by the photon cavity. Furthermore, the current in the photon-induced side-peaks can be strongly enhanced by increasing the electron-photon coupling strength for the case of photons polarized along the transport direction.",
arxiv = "http://arxiv.org/abs/1403.0382"
}

96. Vidar Gudmundsson, Sigtryggur Hauksson, Arni Johnsen, Gilbert Reinisch, Andrei Manolescu, Christophe Besse and Guillaume Dujardin.
Excitation of radial collective modes in a quantum dot: Beyond linear response.
Annalen der Physik 526, 235–248 (2014).
Abstract The recent results on the linear breathing mode of the excitation spectrum of a quantum dot obtained by McDonald et. al [Phys. Rev. Lett. 111, 256801 (2013)] are extended to the nonlinear regime. To accomplish this and analyze the results the response of five different models of two interacting electrons in a quantum dot to an external short lived radial excitation that is strong enough to excite the system well beyond the linear response regime is compared. The models considered describe the Coulomb interaction between the electrons in different ways ranging from mean-field approaches to configuration interaction (CI) models, where the two-electron Hamiltonian is diagonalized in a large truncated Fock space. The radially symmetric excitation is selected in order to severely put to test the different approaches to describe the interaction and correlations of an electron system in a nonequilibrium state. As can be expected for the case of only two electrons none of the mean-field models can in full details reproduce the results obtained by the CI model. Nonetheless, some linear and nonlinear characteristics are reproduced reasonably well. All the models show activation of an increasing number of collective modes as the strength of the excitation is increased. By varying slightly the confinement potential of the dot it was observed how sensitive the properties of the excitation spectrum are to the Coulomb interaction and its correlation effects. In order to approach closer the question of nonlinearity one of the mean-field models has been solved directly in a nonlinear fashion without resorting to iterations.
URL arXiv, DOI BibTeX

@article{ANDP:ANDP201400048,
author = "Gudmundsson, Vidar and Hauksson, Sigtryggur and Johnsen, Arni and Reinisch, Gilbert and Manolescu, Andrei and Besse, Christophe and Dujardin, Guillaume",
title = "Excitation of radial collective modes in a quantum dot: Beyond linear response",
journal = "Annalen der Physik",
volume = 526,
number = "5-6",
issn = "1521-3889",
url = "http://dx.doi.org/10.1002/andp.201400048",
doi = "10.1002/andp.201400048",
pages = "235--248",
year = 2014,
abstract = "The recent results on the linear breathing mode of the excitation spectrum of a quantum dot obtained by McDonald et. al [Phys. Rev. Lett. 111, 256801 (2013)] are extended to the nonlinear regime. To accomplish this and analyze the results the response of five different models of two interacting electrons in a quantum dot to an external short lived radial excitation that is strong enough to excite the system well beyond the linear response regime is compared. The models considered describe the Coulomb interaction between the electrons in different ways ranging from mean-field approaches to configuration interaction (CI) models, where the two-electron Hamiltonian is diagonalized in a large truncated Fock space. The radially symmetric excitation is selected in order to severely put to test the different approaches to describe the interaction and correlations of an electron system in a nonequilibrium state. As can be expected for the case of only two electrons none of the mean-field models can in full details reproduce the results obtained by the CI model. Nonetheless, some linear and nonlinear characteristics are reproduced reasonably well. All the models show activation of an increasing number of collective modes as the strength of the excitation is increased. By varying slightly the confinement potential of the dot it was observed how sensitive the properties of the excitation spectrum are to the Coulomb interaction and its correlation effects. In order to approach closer the question of nonlinearity one of the mean-field models has been solved directly in a nonlinear fashion without resorting to iterations.",
arxiv = "http://arxiv.org/abs/1311.3252"
}

97. A Manolescu, D C Marinescu and T D Stanescu.
Coulomb interaction effects on the Majorana states in quantum wires.
Journal of Physics: Condensed Matter 26, 172203 (2014).
Abstract The stability of the Majorana modes in the presence of a repulsive interaction is studied in the standard semiconductor wire–metallic superconductor configuration. The effects of short-range Coulomb interaction, which is incorporated using a purely repulsive δ -function to model the strong screening effect due to the presence of the superconductor, are determined within a Hartree–Fock approximation of the effective Bogoliubov–De Gennes Hamiltonian that describes the low-energy physics of the wire. Through a numerical diagonalization procedure we obtain interaction corrections to the single particle eigenstates and calculate the extended topological phase diagram in terms of the chemical potential and the Zeeman energy. We find that, for a fixed Zeeman energy, the interaction shifts the phase boundaries to a higher chemical potential, whereas for a fixed chemical potential this shift can occur either at lower or higher Zeeman energies. These effects can be interpreted as a renormalization of the g-factor due to the interaction. The minimum Zeeman energy needed to realize Majorana fermions decreases with the increasing strength of the Coulomb repulsion. Furthermore, we find that in wires with multi-band occupancy this effect can be enhanced by increasing the chemical potential, i.e. by occupying higher energy bands.
URL arXiv, DOI BibTeX

@article{0953-8984-26-17-172203,
author = "A Manolescu and D C Marinescu and T D Stanescu",
title = "Coulomb interaction effects on the Majorana states in quantum wires",
journal = "Journal of Physics: Condensed Matter",
volume = 26,
number = 17,
pages = 172203,
url = "http://stacks.iop.org/0953-8984/26/i=17/a=172203",
year = 2014,
doi = "10.1088/0953-8984/26/17/172203",
abstract = "The stability of the Majorana modes in the presence of a repulsive interaction is studied in the standard semiconductor wire–metallic superconductor configuration. The effects of short-range Coulomb interaction, which is incorporated using a purely repulsive δ -function to model the strong screening effect due to the presence of the superconductor, are determined within a Hartree–Fock approximation of the effective Bogoliubov–De Gennes Hamiltonian that describes the low-energy physics of the wire. Through a numerical diagonalization procedure we obtain interaction corrections to the single particle eigenstates and calculate the extended topological phase diagram in terms of the chemical potential and the Zeeman energy. We find that, for a fixed Zeeman energy, the interaction shifts the phase boundaries to a higher chemical potential, whereas for a fixed chemical potential this shift can occur either at lower or higher Zeeman energies. These effects can be interpreted as a renormalization of the g-factor due to the interaction. The minimum Zeeman energy needed to realize Majorana fermions decreases with the increasing strength of the Coulomb repulsion. Furthermore, we find that in wires with multi-band occupancy this effect can be enhanced by increasing the chemical potential, i.e. by occupying higher energy bands.",
arxiv = "http://arxiv.org/abs/1312.3888"
}

98. Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions.
The European Physical Journal B 87 (2014).
Abstract We calculate the persistent charge and spin polarization current inside a finite-width quantum ring of realistic geometry as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. The time evolution in the transient regime of the two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads is governed by a time-convolutionless non-Markovian generalized master equation. The electrons are correlated via Coulomb interaction. In addition, the ring is embedded in a photon cavity with a single mode of linearly polarized photon field, which is polarized either perpendicular or parallel to the charge transport direction. To analyze carefully the physical effects, we compare to the analytical results of the toy model of a one-dimensional (1D) ring of non-interacting electrons with spin-orbit coupling. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase ΔΦ = π, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin polarization currents of the 1D and 2D ring. Quantitatively, however, the spin polarization currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin polarization current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring, which is embedded in the photon cavity.
URL arXiv, DOI BibTeX

@article{Arnold50144,
year = 2014,
issn = "1434-6028",
journal = "The European Physical Journal B",
eid = 113,
volume = 87,
number = 5,
doi = "10.1140/epjb/e2014-50144-y",
title = "Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions",
url = "http://dx.doi.org/10.1140/epjb/e2014-50144-y",
publisher = "Springer Berlin Heidelberg",
keywords = "Mesoscopic and Nanoscale Systems",
author = "Arnold, Thorsten and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
language = "English",
abstract = "We calculate the persistent charge and spin polarization current inside a finite-width quantum ring of realistic geometry as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. The time evolution in the transient regime of the two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads is governed by a time-convolutionless non-Markovian generalized master equation. The electrons are correlated via Coulomb interaction. In addition, the ring is embedded in a photon cavity with a single mode of linearly polarized photon field, which is polarized either perpendicular or parallel to the charge transport direction. To analyze carefully the physical effects, we compare to the analytical results of the toy model of a one-dimensional (1D) ring of non-interacting electrons with spin-orbit coupling. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase ΔΦ = π, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin polarization currents of the 1D and 2D ring. Quantitatively, however, the spin polarization currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin polarization current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring, which is embedded in the photon cavity.",
arxiv = "http://arxiv.org/abs/1310.5870"
}

99. Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Impact of a circularly polarized cavity photon field on the charge and spin flow through an Aharonov–Casher ring.
Physica E: Low-dimensional Systems and Nanostructures 60, 170 - 182 (2014).
Abstract Abstract We explore the influence of a circularly polarized cavity photon field on the transport properties of a finite-width ring, in which the electrons are subject to spin–orbit and Coulomb interaction. The quantum ring is embedded in an electromagnetic cavity and described by “exact” numerical diagonalization. We study the case that the cavity photon field is circularly polarized and compare it to the linearly polarized case. The quantum device is moreover coupled to external, electrically biased leads. The time propagation in the transient regime is described by a non-Markovian generalized master equation. We find that the spin polarization and the spin photocurrents of the quantum ring are largest for circularly polarized photon field and destructive Aharonov–Casher (AC) phase interference. The charge current suppression dip due to the destructive AC phase becomes threefold under the circularly polarized photon field as the interaction of the electrons׳ angular momentum and spin angular momentum of light causes many-body level splitting leading to three many-body level crossing locations instead of one. The circular charge current inside the ring, which is induced by the circularly polarized photon field, is found to be suppressed in a much wider range around the destructive AC phase than the lead-device-lead charge current. The charge current can be directed through one of the two ring arms with the help of the circularly polarized photon field, but is superimposed by vortices of smaller scale. Unlike the charge photocurrent, the flow direction of the spin photocurrent is found to be independent of the handedness of the circularly polarized photon field.
URL arXiv, DOI BibTeX

@article{Arnold2014170,
title = "Impact of a circularly polarized cavity photon field on the charge and spin flow through an Aharonov–Casher ring",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 60,
number = 0,
pages = "170 - 182",
year = 2014,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2014.02.024",
url = "http://www.sciencedirect.com/science/article/pii/S138694771400085X",
author = "Thorsten Arnold and Chi-Shung Tang and Andrei Manolescu and Vidar Gudmundsson",
keywords = "Circularly polarized photon field",
abstract = "Abstract We explore the influence of a circularly polarized cavity photon field on the transport properties of a finite-width ring, in which the electrons are subject to spin–orbit and Coulomb interaction. The quantum ring is embedded in an electromagnetic cavity and described by “exact” numerical diagonalization. We study the case that the cavity photon field is circularly polarized and compare it to the linearly polarized case. The quantum device is moreover coupled to external, electrically biased leads. The time propagation in the transient regime is described by a non-Markovian generalized master equation. We find that the spin polarization and the spin photocurrents of the quantum ring are largest for circularly polarized photon field and destructive Aharonov–Casher (AC) phase interference. The charge current suppression dip due to the destructive \{AC\} phase becomes threefold under the circularly polarized photon field as the interaction of the electrons׳ angular momentum and spin angular momentum of light causes many-body level splitting leading to three many-body level crossing locations instead of one. The circular charge current inside the ring, which is induced by the circularly polarized photon field, is found to be suppressed in a much wider range around the destructive \{AC\} phase than the lead-device-lead charge current. The charge current can be directed through one of the two ring arms with the help of the circularly polarized photon field, but is superimposed by vortices of smaller scale. Unlike the charge photocurrent, the flow direction of the spin photocurrent is found to be independent of the handedness of the circularly polarized photon field.",
arxiv = "http://arxiv.org/abs/1311.3235"
}

100. Gilbert Reinisch, Vidar Gudmundsson and Andrei Manolescu.
Coherent nonlinear quantum model for composite fermions.
Physics Letters A 378, 1566 - 1570 (2014).
Abstract Abstract Originally proposed by Read [1] and Jain [2], the so-called “composite-fermion” is a phenomenological quasi-particle resulting from the attachment of two local flux quanta, seen as nonlocal vortices, to electrons situated on a two-dimensional (2D) surface embedded in a strong orthogonal magnetic field. In this Letter this phenomenon is described as a highly-nonlinear and coherent mean-field quantum process of the soliton type by use of a 2D stationary Schrödinger–Poisson differential model with only two Coulomb-interacting electrons. At filling factor ν = 1 3 of the lowest Landau level the solution agrees with both the exact two-electron antisymmetric Schrödinger wavefunction and with Laughlin's Jastrow-type guess for the fractional quantum Hall effect, hence providing this latter with a tentative physical justification deduced from the experimental results and based on first principles.
URL arXiv, DOI BibTeX

@article{Reinisch20141566,
title = "Coherent nonlinear quantum model for composite fermions",
journal = "Physics Letters A",
volume = 378,
number = 21,
pages = "1566 - 1570",
year = 2014,
note = "",
issn = "0375-9601",
doi = "10.1016/j.physleta.2014.03.050",
url = "http://www.sciencedirect.com/science/article/pii/S0375960114003272",
author = "Gilbert Reinisch and Vidar Gudmundsson and Andrei Manolescu",
keywords = "Quasi-particles",
abstract = "Abstract Originally proposed by Read [1] and Jain [2], the so-called “composite-fermion” is a phenomenological quasi-particle resulting from the attachment of two local flux quanta, seen as nonlocal vortices, to electrons situated on a two-dimensional (2D) surface embedded in a strong orthogonal magnetic field. In this Letter this phenomenon is described as a highly-nonlinear and coherent mean-field quantum process of the soliton type by use of a 2D stationary Schrödinger–Poisson differential model with only two Coulomb-interacting electrons. At filling factor ν = 1 3 of the lowest Landau level the solution agrees with both the exact two-electron antisymmetric Schrödinger wavefunction and with Laughlin's Jastrow-type guess for the fractional quantum Hall effect, hence providing this latter with a tentative physical justification deduced from the experimental results and based on first principles.",
arxiv = "http://arxiv.org/abs/1306.6869"
}

101. Tomas Orn Rosdahl, Andrei Manolescu and Vidar Gudmundsson.
Spin and impurity effects on flux-periodic oscillations in core-shell nanowires.
Phys. Rev. B 90, 035421 (July 2014).
Abstract We study the quantum mechanical states of electrons situated on a cylindrical surface of finite axial length to model a semiconductor core-shell nanowire. We calculate the conductance in the presence of a longitudinal magnetic field by weakly coupling the cylinder to semi-infinite leads. Spin effects are accounted for through Zeeman coupling and Rashba spin-orbit interaction (SOI). Emphasis is on manifestations of flux-periodic oscillations and we show how factors such as impurities, contact geometry, and spin affect them. Oscillations survive and remain periodic in the presence of impurities, noncircular contacts, and SOI, while Zeeman splitting results in aperiodicity, beating patterns, and additional background fluctuations. Our results are in qualitative agreement with recent magnetotransport experiments performed on GaAs/InAs core-shell nanowires. Lastly, we propose methods of data analysis for detecting the presence of Rashba SOI in core-shell systems and for estimating the electron g factor in the shell.
URL arXiv, DOI BibTeX

@article{PhysRevB.90.035421,
title = "Spin and impurity effects on flux-periodic oscillations in core-shell nanowires",
author = "Rosdahl, Tomas Orn and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 90,
issue = 3,
pages = 035421,
numpages = 11,
year = 2014,
month = "Jul",
publisher = "American Physical Society",
doi = "10.1103/PhysRevB.90.035421",
abstract = "We study the quantum mechanical states of electrons situated on a cylindrical surface of finite axial length to model a semiconductor core-shell nanowire. We calculate the conductance in the presence of a longitudinal magnetic field by weakly coupling the cylinder to semi-infinite leads. Spin effects are accounted for through Zeeman coupling and Rashba spin-orbit interaction (SOI). Emphasis is on manifestations of flux-periodic oscillations and we show how factors such as impurities, contact geometry, and spin affect them. Oscillations survive and remain periodic in the presence of impurities, noncircular contacts, and SOI, while Zeeman splitting results in aperiodicity, beating patterns, and additional background fluctuations. Our results are in qualitative agreement with recent magnetotransport experiments performed on GaAs/InAs core-shell nanowires. Lastly, we propose methods of data analysis for detecting the presence of Rashba SOI in core-shell systems and for estimating the electron g factor in the shell.",
arxiv = "http://arxiv.org/abs/1404.1798"
}

102. Kristinn Torfason, Andrei Manolescu, Sigurdur I Erlingsson and Vidar Gudmundsson.
Thermoelectric current and Coulomb-blockade plateaus in a quantum dot.
Physica E: Low-dimensional Systems and Nanostructures 53, 178 - 185 (2013).
Abstract A Generalized Master Equation (GME) is used to study the thermoelectric currents through a quantum dot in both the transient and steady-state regime. The two semi-infinite leads are kept at the same chemical potential but at different temperatures to produce a thermoelectric current which has a varying sign depending on the chemical potential. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. We observe a saw-teeth like profile of the current alternating with plateaus of almost zero current. Our calculations go beyond the linear response with respect to the temperature gradient, but are compatible with known results for the thermopower in the linear response regime.
URL arXiv, DOI BibTeX

@article{Torfason2013178,
title = "Thermoelectric current and Coulomb-blockade plateaus in a quantum dot",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 53,
number = 0,
pages = "178 - 185",
year = 2013,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2013.05.005",
url = "http://www.sciencedirect.com/science/article/pii/S1386947713001689",
author = "Kristinn Torfason and Andrei Manolescu and Sigurdur I. Erlingsson and Vidar Gudmundsson",
abstract = "A Generalized Master Equation (GME) is used to study the thermoelectric currents through a quantum dot in both the transient and steady-state regime. The two semi-infinite leads are kept at the same chemical potential but at different temperatures to produce a thermoelectric current which has a varying sign depending on the chemical potential. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. We observe a saw-teeth like profile of the current alternating with plateaus of almost zero current. Our calculations go beyond the linear response with respect to the temperature gradient, but are compatible with known results for the thermopower in the linear response regime.",
arxiv = "http://arxiv.org/abs/1303.3160"
}

103. Olafur Jonasson, Vidar Gudmundsson, Andrei Manolescu, Chi-Shung Tang and Hsi-Sheng Goan.
Symmetric excitation and de-excitation of a cavity QED system.
The European Physical Journal B 86, 1-7.
Abstract We calculate the time evolution of a cavity-QED system subject to a time dependent sinusoidal drive. The drive is modulated by an envelope function with the shape of a pulse. The system consists of electrons embedded in a semiconductor nanostructure which is coupled to a single mode quantized electromagnetic field. The electron-electron as well as photon-electron interaction is treated exactly using exact numerical diagonalization and the time evolution is calculated by numerically solving the equation of motion for the system's density matrix. We find that the drive causes symmetric excitation and de-excitation where the system climbs up the Jaynes-Cummings ladder and descends back down symmetrically into its original state. This effect is known at low electron-photon coupling strengths but our main finding is how robust the effect is even at ultra-strong coupling strength where the JC-model does not give qualitatively correct results. We investigate the robustness of this symmetric behavior with respect to the drive de-tuning and pulse duration.
URL arXiv, DOI BibTeX

@article{,
jonasson2013 year = 2013,
issn = "1434-6028",
journal = "The European Physical Journal B",
volume = 86,
number = 6,
doi = "10.1140/epjb/e2013-40330-x",
title = "Symmetric excitation and de-excitation of a cavity QED system",
url = "http://dx.doi.org/10.1140/epjb/e2013-40330-x",
publisher = "Springer-Verlag",
keywords = "Mesoscopic and Nanoscale Systems",
author = "Jonasson, Olafur and Gudmundsson, Vidar and Manolescu, Andrei and Tang, Chi-Shung and Goan, Hsi-Sheng",
pages = "1-7",
language = "English",
abstract = "We calculate the time evolution of a cavity-QED system subject to a time dependent sinusoidal drive. The drive is modulated by an envelope function with the shape of a pulse. The system consists of electrons embedded in a semiconductor nanostructure which is coupled to a single mode quantized electromagnetic field. The electron-electron as well as photon-electron interaction is treated exactly using exact numerical diagonalization and the time evolution is calculated by numerically solving the equation of motion for the system's density matrix. We find that the drive causes symmetric excitation and de-excitation where the system climbs up the Jaynes-Cummings ladder and descends back down symmetrically into its original state. This effect is known at low electron-photon coupling strengths but our main finding is how robust the effect is even at ultra-strong coupling strength where the JC-model does not give qualitatively correct results. We investigate the robustness of this symmetric behavior with respect to the drive de-tuning and pulse duration.",
arxiv = "http://arxiv.org/abs/1207.6797"
}

104. P Jonsson, Marjan Ilkov, A Manolescu, A Pedersen and A Valfells.
Tunability of the terahertz space-charge modulation in a vacuum microdiode.
Physics of Plasmas 20, 023107 (2013).
Abstract Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.
URL arXiv, DOI BibTeX

@article{jonsson:023107,
author = "P. Jonsson and Marjan Ilkov and A. Manolescu and A. Pedersen and A. Valfells",
collaboration = "",
title = "Tunability of the terahertz space-charge modulation in a vacuum microdiode",
publisher = "AIP",
year = 2013,
journal = "Physics of Plasmas",
volume = 20,
number = 2,
eid = 023107,
numpages = 7,
pages = 023107,
keywords = "molecular dynamics method; plasma diodes; plasma oscillations; plasma simulation; plasma transport processes; space charge",
doi = "10.1063/1.4793451",
abstract = "Under certain conditions, space-charge limited emission in vacuum microdiodes manifests as clearly defined bunches of charge with a regular size and interval. The frequency corresponding to this interval is in the terahertz range. In this computational study, it is demonstrated that, for a range of parameters, conducive to generating THz frequency oscillations, the frequency is dependant only on the cold cathode electric field and on the emitter area. For a planar micro-diode of given dimension, the modulation frequency can be easily tuned simply by varying the applied potential. Simulations of the microdiode are done for 84 different combinations of emitter area, applied voltage, and gap spacing, using a molecular dynamics based code with exact Coulomb interaction between all electrons in the vacuum gap, which is of the order 100. It is found, for a fixed emitter area, that the frequency of the pulse train is solely dependant on the vacuum electric field in the diode, described by a simple power law. It is also found that, for a fixed value of the electric field, the frequency increases with diminishing size of the emitting spot on the cathode. Some observations are made on the spectral quality, and how it is affected by the gap spacing in the diode and the initial velocity of the electrons.",
arxiv = "http://arxiv.org/abs/1301.6533"
}

105. V Gudmundsson, O Jonasson, Th. Arnold, C-S Tang, H -S Goan and A Manolescu.
Stepwise introduction of model complexity in a generalized master equation approach to time-dependent transport.
Fortschritte der Physik 61, 305–316 (2013).
Abstract We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using exact numerical diagonalization.
URL arXiv, DOI BibTeX

@article{PROP:PROP201200053,
author = "Gudmundsson, V. and Jonasson, O. and Arnold, Th. and Tang, C-S. and Goan, H.-S. and Manolescu, A.",
title = "Stepwise introduction of model complexity in a generalized master equation approach to time-dependent transport",
journal = "Fortschritte der Physik",
volume = 61,
number = "2-3",
publisher = "WILEY-VCH Verlag",
issn = "1521-3978",
url = "http://dx.doi.org/10.1002/prop.201200053",
doi = "10.1002/prop.201200053",
pages = "305--316",
keywords = "Open system, Coulomb interaction, photon cavity, time-dependent transport.",
year = 2013,
abstract = "We demonstrate that with a stepwise introduction of complexity to a model of an electron system embedded in a photonic cavity and a carefully controlled stepwise truncation of the ensuing many-body space it is possible to describe the time-dependent transport of electrons through the system with a non-Markovian generalized quantum master equation. We show how this approach retains effects of the geometry of an anisotropic electronic system. The Coulomb interaction between the electrons and the full electromagnetic coupling between the electrons and the photons are treated in a non-perturbative way using exact numerical diagonalization.",
arxiv = "http://arxiv.org/abs/1203.3048"
}

106. Thorsten Arnold, Chi-Shung Tang, Andrei Manolescu and Vidar Gudmundsson.
Magnetic-field-influenced nonequilibrium transport through a quantum ring with correlated electrons in a photon cavity.
Phys. Rev. B 87, 035314 (January 2013).
Abstract We investigate magnetic-field-influenced time-dependent transport of Coulomb interacting electrons through a two-dimensional quantum ring in an electromagnetic cavity under nonequilibrium conditions described by a time-convolutionless non-Markovian master equation formalism. We take into account the full electromagnetic interaction of electrons and cavity photons. A bias voltage is applied to semi-infinite leads along the x axis, which are connected to the quantum ring. The magnetic field is tunable to manipulate the time-dependent electron transport coupled to a photon field with either x or y polarization. We find that the lead-system-lead current is strongly suppressed by the y-polarized photon field at magnetic field with two flux quanta due to a degeneracy of the many-body energy spectrum of the mostly occupied states. On the other hand, the lead-system-lead current can be significantly enhanced by the y-polarized field at magnetic field with half-integer flux quanta. Furthermore, the y- polarized photon field perturbs the periodicity of the persistent current with the magnetic field and suppresses the magnitude of the persistent current. The spatial and temporal density distributions reflect the characteristics of the many-body spectrum. The vortex formation in the contact areas to the leads influences the charge circulation in the ring.
URL arXiv, DOI BibTeX

@article{PhysRevB.87.035314,
title = "Magnetic-field-influenced nonequilibrium transport through a quantum ring with correlated electrons in a photon cavity",
author = "Arnold, Thorsten and Tang, Chi-Shung and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 87,
issue = 3,
pages = 035314,
numpages = 13,
year = 2013,
month = "Jan",
doi = "10.1103/PhysRevB.87.035314",
publisher = "American Physical Society",
abstract = "We investigate magnetic-field-influenced time-dependent transport of Coulomb interacting electrons through a two-dimensional quantum ring in an electromagnetic cavity under nonequilibrium conditions described by a time-convolutionless non-Markovian master equation formalism. We take into account the full electromagnetic interaction of electrons and cavity photons. A bias voltage is applied to semi-infinite leads along the x axis, which are connected to the quantum ring. The magnetic field is tunable to manipulate the time-dependent electron transport coupled to a photon field with either x or y polarization. We find that the lead-system-lead current is strongly suppressed by the y-polarized photon field at magnetic field with two flux quanta due to a degeneracy of the many-body energy spectrum of the mostly occupied states. On the other hand, the lead-system-lead current can be significantly enhanced by the y-polarized field at magnetic field with half-integer flux quanta. Furthermore, the y- polarized photon field perturbs the periodicity of the persistent current with the magnetic field and suppresses the magnitude of the persistent current. The spatial and temporal density distributions reflect the characteristics of the many-body spectrum. The vortex formation in the contact areas to the leads influences the charge circulation in the ring.",
arxiv = "http://arxiv.org/abs/1209.2888"
}

107. Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan, Andrei Manolescu and Vidar Gudmundsson.
Nonperturbative approach to circuit quantum electrodynamics.
Phys. Rev. E 86, 046701 (October 2012).
Abstract We outline a rigorous method which can be used to solve the many-body Schrödinger equation for a Coulomb interacting electronic system in an external classical magnetic field as well as a quantized electromagnetic field. Effects of the geometry of the electronic system as well as the polarization of the quantized electromagnetic field are explicitly taken into account. We accomplish this by performing repeated truncations of many-body spaces in order to keep the size of the many particle basis on a manageable level. The electron-electron and electron-photon interactions are treated in a nonperturbative manner using “exact numerical diagonalization.” Our results demonstrate that including the diamagnetic term in the photon-electron interaction Hamiltonian drastically improves numerical convergence. Additionally, convergence with respect to the number of photon states in the joint photon-electron Fock space basis is fast. However, the convergence with respect to the number of electronic states is slow and is the main bottleneck in calculations.
URL arXiv, DOI BibTeX

@article{PhysRevE.86.046701,
title = "Nonperturbative approach to circuit quantum electrodynamics",
author = "Jonasson, Olafur and Tang, Chi-Shung and Goan, Hsi-Sheng and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. E",
volume = 86,
issue = 4,
pages = 046701,
numpages = 8,
year = 2012,
month = "Oct",
doi = "10.1103/PhysRevE.86.046701",
publisher = "American Physical Society",
abstract = "We outline a rigorous method which can be used to solve the many-body Schrödinger equation for a Coulomb interacting electronic system in an external classical magnetic field as well as a quantized electromagnetic field. Effects of the geometry of the electronic system as well as the polarization of the quantized electromagnetic field are explicitly taken into account. We accomplish this by performing repeated truncations of many-body spaces in order to keep the size of the many particle basis on a manageable level. The electron-electron and electron-photon interactions are treated in a nonperturbative manner using “exact numerical diagonalization.” Our results demonstrate that including the diamagnetic term in the photon-electron interaction Hamiltonian drastically improves numerical convergence. Additionally, convergence with respect to the number of photon states in the joint photon-electron Fock space basis is fast. However, the convergence with respect to the number of electronic states is slow and is the main bottleneck in calculations.",
arxiv = "http://arxiv.org/abs/1203.5980"
}

108. Marian Niţă, D C Marinescu, Andrei Manolescu, Bogdan Ostahie and Vidar Gudmundsson.
Persistent oscillatory currents in a 1D ring with Rashba and Dresselhaus spin–orbit interactions excited by a terahertz pulse.
Physica E: Low-dimensional Systems and Nanostructures 46, 12 - 20 (2012).
Abstract Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba and Dresselhaus spin–orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability.
URL arXiv, DOI BibTeX

@article{Niţă201212,
title = "Persistent oscillatory currents in a 1D ring with Rashba and Dresselhaus spin–orbit interactions excited by a terahertz pulse",
journal = "Physica E: Low-dimensional Systems and Nanostructures",
volume = 46,
number = 0,
pages = "12 - 20",
year = 2012,
note = "",
issn = "1386-9477",
doi = "10.1016/j.physe.2012.08.017",
url = "http://www.sciencedirect.com/science/article/pii/S1386947712003232",
author = "Marian Niţă and D.C. Marinescu and Andrei Manolescu and Bogdan Ostahie and Vidar Gudmundsson",
abstract = "Persistent, oscillatory charge and spin currents are shown to be driven by a two-component terahertz laser pulse in a one-dimensional mesoscopic ring with Rashba and Dresselhaus spin–orbit interactions (SOI) linear in the electron momentum. The characteristic interference effects result from the opposite precession directions imposed on the electron spin by the two SOI couplings. The time dependence of the currents is obtained by solving numerically the equation of motion for the density operator, which is later employed in calculating statistical averages of quantum operators on few electron eigenstates. The parameterization of the problem is done in terms of the SOI coupling constants and of the phase difference between the two laser components. Our results indicate that the amplitude of the oscillations is controlled by the relative strength of the two SOI's, while their frequency is determined by the difference between the excitation energies of the electron states. Furthermore, the oscillations of the spin current acquire a beating pattern of higher frequency that we associate with the nutation of the electron spin between the quantization axes of the two SOI couplings. This phenomenon disappears at equal SOI strengths, whereby the opposite precessions occur with the same probability.",
arxiv = "http://arxiv.org/abs/1111.2949"
}

109. Kristinn Torfason, Andrei Manolescu, Valeriu Molodoveanu and Vidar Gudmundsson.
Excitation of collective modes in a quantum flute.
Phys. Rev. B 85, 245114 (June 2012).
Abstract We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum “flute” device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.
URL arXiv, DOI BibTeX

@article{PhysRevB.85.245114,
title = "Excitation of collective modes in a quantum flute",
author = "Torfason, Kristinn and Manolescu, Andrei and Molodoveanu, Valeriu and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 85,
issue = 24,
pages = 245114,
numpages = 9,
year = 2012,
month = "Jun",
doi = "10.1103/PhysRevB.85.245114",
publisher = "American Physical Society",
abstract = "We use a generalized master equation (GME) formalism to describe the nonequilibrium time-dependent transport of Coulomb interacting electrons through a short quantum wire connected to semi-infinite biased leads. The contact strength between the leads and the wire is modulated by out-of-phase time-dependent potentials that simulate a turnstile device. We explore this setup by keeping the contact with one lead at a fixed location at one end of the wire, whereas the contact with the other lead is placed on various sites along the length of the wire. We study the propagation of sinusoidal and rectangular pulses. We find that the current profiles in both leads depend not only on the shape of the pulses, but also on the position of the second contact. The current reflects standing waves created by the contact potentials, like in a wind musical instrument (for example, a flute), but occurring on the background of the equilibrium charge distribution. The number of electrons in our quantum “flute” device varies between two and three. We find that for rectangular pulses the currents in the leads may flow against the bias for short time intervals, due to the higher harmonics of the charge response. The GME is solved numerically in small time steps without resorting to the traditional Markov and rotating wave approximations. The Coulomb interaction between the electrons in the sample is included via the exact diagonalization method. The system (leads plus sample wire) is described by a lattice model.",
arxiv = "http://arxiv.org/abs/1202.0566"
}

110. D C Marinescu and Andrei Manolescu.
Weak localization in a lateral superlattice with Rashba and Dresselhaus spin-orbit interaction.
Phys. Rev. B 85, 165302 (April 2012).
Abstract We calculate the weak localization (WL) correction to the conductivity of a lateral superlattice (LSL) with Rashba (R)-Dresselhaus (D) spin-orbit interaction (SOI). The superlattice is modeled as a sequence of parallel wires that support tunneling between adjacent sites, leading to the formation of extended Bloch states along its axis and a miniband in the energy spectrum. Our results, obtained by calculating the eigenvalues of the Cooperon operator in the diffusion approximation, indicate that the electron dephasing rate that determines the antilocalization correction is enhanced by a term proportional with the LSL potential and the bandwidth. Within the same formalism, the spin-relaxation rates associated with the localization corrections are found to exhibit a strong anisotropy dictated by the relative strength of the two SOI couplings, as well as by the orientation of the LSL axis.
URL, DOI BibTeX

@article{PhysRevB.85.165302,
title = "Weak localization in a lateral superlattice with Rashba and Dresselhaus spin-orbit interaction",
author = "Marinescu, D. C. and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 85,
issue = 16,
pages = 165302,
numpages = 8,
year = 2012,
month = "Apr",
doi = "10.1103/PhysRevB.85.165302",
abstract = "We calculate the weak localization (WL) correction to the conductivity of a lateral superlattice (LSL) with Rashba (R)-Dresselhaus (D) spin-orbit interaction (SOI). The superlattice is modeled as a sequence of parallel wires that support tunneling between adjacent sites, leading to the formation of extended Bloch states along its axis and a miniband in the energy spectrum. Our results, obtained by calculating the eigenvalues of the Cooperon operator in the diffusion approximation, indicate that the electron dephasing rate that determines the antilocalization correction is enhanced by a term proportional with the LSL potential and the bandwidth. Within the same formalism, the spin-relaxation rates associated with the localization corrections are found to exhibit a strong anisotropy dictated by the relative strength of the two SOI couplings, as well as by the orientation of the LSL axis.",
publisher = "American Physical Society"
}

111. Vidar Gudmundsson, Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan and Andrei Manolescu.
Time-dependent transport of electrons through a photon cavity.
Phys. Rev. B 85, 075306 (February 2012).
Abstract We use a non-Markovian master equation to describe the transport of Coulomb-interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite-parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at t=0. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces, we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating-wave approximation or reduction of the electron states to two levels into account. We observe that the number of initial cavity photons and their polarizations can have important effects on the transport properties of the system. The quasiparticles formed in the central system have lifetimes limited by the coupling to the leads and radiation processes active on a much longer time scale.
URL arXiv, DOI BibTeX

@article{PhysRevB.85.075306,
title = "Time-dependent transport of electrons through a photon cavity",
author = "Gudmundsson, Vidar and Jonasson, Olafur and Tang, Chi-Shung and Goan, Hsi-Sheng and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 85,
issue = 7,
pages = 075306,
numpages = 13,
year = 2012,
month = "Feb",
doi = "10.1103/PhysRevB.85.075306",
publisher = "American Physical Society",
abstract = "We use a non-Markovian master equation to describe the transport of Coulomb-interacting electrons through an electromagnetic cavity with one quantized photon mode. The central system is a finite-parabolic quantum wire that is coupled weakly to external parabolic quasi-one-dimensional leads at t=0. With a stepwise introduction of complexity to the description of the system and a corresponding stepwise truncation of the ensuing many-body spaces, we are able to describe the time-dependent transport of Coulomb-interacting electrons through a geometrically complex central system. We take the full electromagnetic interaction of electrons and cavity photons without resorting to the rotating-wave approximation or reduction of the electron states to two levels into account. We observe that the number of initial cavity photons and their polarizations can have important effects on the transport properties of the system. The quasiparticles formed in the central system have lifetimes limited by the coupling to the leads and radiation processes active on a much longer time scale.",
arxiv = "http://arxiv.org/abs/1109.4728"
}

112. Olafur Jonasson, Chi-Shung Tang, Hsi-Sheng Goan, Andrei Manolescu and Vidar Gudmundsson.
Quantum magneto-electrodynamics of electrons embedded in a photon cavity.
New Journal of Physics 14, 013036 (2012).
Abstract We investigate the coupling between a quantized electromagnetic field in a cavity resonator and a Coulomb interacting electronic system in a nanostructure in an external magnetic field. The effects caused by the geometry of the electronic system and the polarization of the electromagnetic field are explicitly taken into account. Our numerical results demonstrate that the two-level system approximation and the Jaynes–Cummings model remain valid in the weak electron–photon coupling regime, while the quadratic vector potential in the diamagnetic part of the charge current leads to significant correction to the energy spectrum in the strong coupling regime. Furthermore, we find that coupling to a strong cavity photon mode polarizes the charge distribution of the system, requiring a large basis of single-electron eigenstates to be included in the model.
URL arXiv BibTeX

@article{1367-2630-14-1-013036,
author = "Olafur Jonasson and Chi-Shung Tang and Hsi-Sheng Goan and Andrei Manolescu and Vidar Gudmundsson",
title = "Quantum magneto-electrodynamics of electrons embedded in a photon cavity",
journal = "New Journal of Physics",
volume = 14,
number = 1,
pages = 013036,
url = "http://stacks.iop.org/1367-2630/14/i=1/a=013036",
year = 2012,
abstract = "We investigate the coupling between a quantized electromagnetic field in a cavity resonator and a Coulomb interacting electronic system in a nanostructure in an external magnetic field. The effects caused by the geometry of the electronic system and the polarization of the electromagnetic field are explicitly taken into account. Our numerical results demonstrate that the two-level system approximation and the Jaynes–Cummings model remain valid in the weak electron–photon coupling regime, while the quadratic vector potential in the diamagnetic part of the charge current leads to significant correction to the energy spectrum in the strong coupling regime. Furthermore, we find that coupling to a strong cavity photon mode polarizes the charge distribution of the system, requiring a large basis of single-electron eigenstates to be included in the model.",
arxiv = "http://arxiv.org/abs/1109.4594"
}

113. Csaba Daday, Andrei Manolescu, D C Marinescu and Vidar Gudmundsson.
Electronic charge and spin density distribution in a quantum ring with spin-orbit and Coulomb interactions.
Phys. Rev. B 84, 115311 (September 2011).
Abstract Charge and spin density distributions are studied within a nanoring structure endowed with Rashba and Dresselhaus spin-orbit interactions (SOIs). For a small number of electrons, in the presence of an external magnetic field and of the Coulomb interaction, the energy spectrum of the system is calculated through an exact numerical diagonalization procedure. The eigenstates thus determined are used to estimate the charge and spin densities around the ring. We find that when more than two electrons are considered, the charge density deformations induced by SOIs are dramatically flattened by the Coulomb repulsion, while the spin density deformations are amplified.
URL arXiv, DOI BibTeX

@article{PhysRevB.84.115311,
title = "Electronic charge and spin density distribution in a quantum ring with spin-orbit and Coulomb interactions",
author = "Daday, Csaba and Manolescu, Andrei and Marinescu, D. C. and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 84,
issue = 11,
pages = 115311,
numpages = 6,
year = 2011,
month = "Sep",
doi = "10.1103/PhysRevB.84.115311",
abstract = "Charge and spin density distributions are studied within a nanoring structure endowed with Rashba and Dresselhaus spin-orbit interactions (SOIs). For a small number of electrons, in the presence of an external magnetic field and of the Coulomb interaction, the energy spectrum of the system is calculated through an exact numerical diagonalization procedure. The eigenstates thus determined are used to estimate the charge and spin densities around the ring. We find that when more than two electrons are considered, the charge density deformations induced by SOIs are dramatically flattened by the Coulomb repulsion, while the spin density deformations are amplified.",
publisher = "American Physical Society",
arxiv = "http://arxiv.org/abs/1106.3697"
}

114. Marian Niţă, D C Marinescu, Andrei Manolescu and Vidar Gudmundsson.
Nonadiabatic generation of a pure spin current in a one-dimensional quantum ring with spin-orbit interaction.
Phys. Rev. B 83, 155427 (April 2011).
Abstract We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spin-orbit interaction by irradiation with a nonadiabatic, two-component terahertz laser pulse, whose spatial asymmetry is reflected by an internal phase difference ϕ. The solutions of the equation of motion for the density operator are obtained for a spin-orbit coupling linear in the electron momentum (Rashba) and they are used to calculate the time-dependent charge and spin currents. We find that there are critical values of ϕ at which the charge current disappears, while the spin current reaches a maximum or a minimum value.
URL arXiv, DOI BibTeX

@article{PhysRevB.83.155427,
title = "Nonadiabatic generation of a pure spin current in a one-dimensional quantum ring with spin-orbit interaction",
author = "Niţă, Marian and Marinescu, D. C. and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 83,
issue = 15,
pages = 155427,
numpages = 5,
year = 2011,
month = "Apr",
doi = "10.1103/PhysRevB.83.155427",
abstract = "We demonstrate the theoretical possibility of obtaining a pure spin current in a 1D ring with spin-orbit interaction by irradiation with a nonadiabatic, two-component terahertz laser pulse, whose spatial asymmetry is reflected by an internal phase difference ϕ. The solutions of the equation of motion for the density operator are obtained for a spin-orbit coupling linear in the electron momentum (Rashba) and they are used to calculate the time-dependent charge and spin currents. We find that there are critical values of ϕ at which the charge current disappears, while the spin current reaches a maximum or a minimum value.",
publisher = "American Physical Society",
arxiv = "http://arxiv.org/abs/1012.4952"
}

115. Vidar Gudmundsson, Chi-Shung Tang, Cosmin Mihai Gainar, Valeriu Moldoveanu and Andrei Manolescu.
Time-dependent magnetotransport in semiconductor nanostructures via the generalized master equation.
Computer Physics Communications 182, 46 - 48 (2011).
Abstract Transport of electrons through two-dimensional semiconductor structures on the nanoscale in the presence of perpendicular magnetic field depends on the interplay of geometry of the system, the leads, and the magnetic length. We use a generalized master equation (GME) formalism to describe the transport through the system without resorting to the Markov approximation. Coupling to the leads results in elastic and inelastic processes in the system that are described to a high order by the integro-differential equation of the GME formalism. Geometrical details of systems and leads leave their fingerprints on the transport of electrons through them. The GME formalism can be used to describe both the initial transient regime immediately after the coupling of the leads to the system and the steady state achieved after a longer time.
URL arXiv, DOI BibTeX

@article{Gudmundsson201146,
title = "Time-dependent magnetotransport in semiconductor nanostructures via the generalized master equation",
journal = "Computer Physics Communications",
volume = 182,
number = 1,
pages = "46 - 48",
year = 2011,
note = "Computer Physics Communications Special Edition for Conference on Computational Physics Kaohsiung, Taiwan, Dec 15-19, 2009",
issn = "0010-4655",
doi = "10.1016/j.cpc.2010.08.006",
url = "http://www.sciencedirect.com/science/article/pii/S0010465510002961",
author = "Vidar Gudmundsson and Chi-Shung Tang and Cosmin Mihai Gainar and Valeriu Moldoveanu and Andrei Manolescu",
arxiv = "http://arxiv.org/abs/1002.1579",
abstract = "Transport of electrons through two-dimensional semiconductor structures on the nanoscale in the presence of perpendicular magnetic field depends on the interplay of geometry of the system, the leads, and the magnetic length. We use a generalized master equation (GME) formalism to describe the transport through the system without resorting to the Markov approximation. Coupling to the leads results in elastic and inelastic processes in the system that are described to a high order by the integro-differential equation of the GME formalism. Geometrical details of systems and leads leave their fingerprints on the transport of electrons through them. The GME formalism can be used to describe both the initial transient regime immediately after the coupling of the leads to the system and the steady state achieved after a longer time.",
keywords = "Generalized master equation"
}

116. Cosmin Mihai Gainar, Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Turnstile pumping through an open quantum wire.
New Journal of Physics 13, 013014 (2011).
Abstract We use a non-Markovian generalized master equation (GME) to describe the time-dependent charge transfer through a parabolically confined quantum wire of a finite length coupled to semi-infinite quasi-two-dimensional (2D) leads. The quantum wire and the leads are in a perpendicular external magnetic field. The contacts to the left and right leads depend on time and are kept out of phase to model a quantum turnstile of finite size. The effects of the driving period of the turnstile, the external magnetic field, the character of the contacts and the chemical potential bias on the effectiveness of the charge transfer of the turnstile are examined, in both the absence and the presence of the magnetic field. The interplay between the strength of the coupling and the strength of the magnetic field is also discussed. We observe how the edge states created in the presence of the magnetic field contribute to the pumped charge.
URL arXiv BibTeX

@article{1367-2630-13-1-013014,
author = "Cosmin Mihai Gainar and Valeriu Moldoveanu and Andrei Manolescu and Vidar Gudmundsson",
title = "Turnstile pumping through an open quantum wire",
journal = "New Journal of Physics",
volume = 13,
number = 1,
pages = 013014,
url = "http://stacks.iop.org/1367-2630/13/i=1/a=013014",
year = 2011,
arxiv = "http://arxiv.org/abs/1004.4052",
abstract = "We use a non-Markovian generalized master equation (GME) to describe the time-dependent charge transfer through a parabolically confined quantum wire of a finite length coupled to semi-infinite quasi-two-dimensional (2D) leads. The quantum wire and the leads are in a perpendicular external magnetic field. The contacts to the left and right leads depend on time and are kept out of phase to model a quantum turnstile of finite size. The effects of the driving period of the turnstile, the external magnetic field, the character of the contacts and the chemical potential bias on the effectiveness of the charge transfer of the turnstile are examined, in both the absence and the presence of the magnetic field. The interplay between the strength of the coupling and the strength of the magnetic field is also discussed. We observe how the edge states created in the presence of the magnetic field contribute to the pumped charge."
}

117. Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Dynamic correlations induced by Coulomb interactions in coupled quantum dots.
Phys. Rev. B 82, 085311 (August 2010).
Abstract Time-dependent transport through two capacitively coupled quantum dots is studied in the framework of the generalized master equation. The Coulomb interaction is included within the exact diagonalization method. Each dot is connected to two leads at different times, such that a steady state is established in one dot before the coupling of the other dot to its leads. By appropriately tuning the bias windows on each dot we find that in the final steady state the transport may be suppressed or enhanced. These two cases are explained by the redistribution of charge on the many-body states built on both dots. We also predict and analyze the transient mutual charge sensing of the dots.
URL, DOI BibTeX

@article{PhysRevB.82.085311,
title = "Dynamic correlations induced by Coulomb interactions in coupled quantum dots",
author = "Moldoveanu, Valeriu and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 82,
issue = 8,
pages = 085311,
numpages = 5,
year = 2010,
month = "Aug",
doi = "10.1103/PhysRevB.82.085311",
abstract = "Time-dependent transport through two capacitively coupled quantum dots is studied in the framework of the generalized master equation. The Coulomb interaction is included within the exact diagonalization method. Each dot is connected to two leads at different times, such that a steady state is established in one dot before the coupling of the other dot to its leads. By appropriately tuning the bias windows on each dot we find that in the final steady state the transport may be suppressed or enhanced. These two cases are explained by the redistribution of charge on the many-body states built on both dots. We also predict and analyze the transient mutual charge sensing of the dots.",
publisher = "American Physical Society"
}

118. Vidar Gudmundsson, Chi-Shung Tang, Olafur Jonasson, Valeriu Moldoveanu and Andrei Manolescu.
Correlated time-dependent transport through a two-dimensional quantum structure.
Phys. Rev. B 81, 205319 (May 2010).
Abstract We use a generalized master equation (GME) to describe the nonequilibrium magnetotransport of interacting electrons through a broad finite quantum wire with an embedded ring structure. The finite quantum wire is weakly coupled to two broad leads acting as reservoirs of electrons. The mutual Coulomb interaction of the electrons is described using a configuration interaction method for the many-electron states of the central system. We report some nontrivial interaction effects both at the level of time-dependent filling of states and on the time-dependent transport. We find that the Coulomb interaction in this nontrivial geometry can enhance the correlation of electronic states in the system and facilitate it’s charging in certain circumstances in the weak coupling limit appropriate for the GME. In addition, we find oscillations in the current in the leads due to the correlations oscillations caused by the switched-on lead-system coupling. The oscillations are influenced and can be enhanced by the external magnetic field and the Coulomb interaction.
URL arXiv, DOI BibTeX

@article{PhysRevB.81.205319,
title = "Correlated time-dependent transport through a two-dimensional quantum structure",
author = "Gudmundsson, Vidar and Tang, Chi-Shung and Jonasson, Olafur and Moldoveanu, Valeriu and Manolescu, Andrei",
journal = "Phys. Rev. B",
volume = 81,
issue = 20,
pages = 205319,
numpages = 11,
year = 2010,
month = "May",
doi = "10.1103/PhysRevB.81.205319",
publisher = "American Physical Society",
abstract = "We use a generalized master equation (GME) to describe the nonequilibrium magnetotransport of interacting electrons through a broad finite quantum wire with an embedded ring structure. The finite quantum wire is weakly coupled to two broad leads acting as reservoirs of electrons. The mutual Coulomb interaction of the electrons is described using a configuration interaction method for the many-electron states of the central system. We report some nontrivial interaction effects both at the level of time-dependent filling of states and on the time-dependent transport. We find that the Coulomb interaction in this nontrivial geometry can enhance the correlation of electronic states in the system and facilitate it’s charging in certain circumstances in the weak coupling limit appropriate for the GME. In addition, we find oscillations in the current in the leads due to the correlations oscillations caused by the switched-on lead-system coupling. The oscillations are influenced and can be enhanced by the external magnetic field and the Coulomb interaction.",
arxiv = "http://arxiv.org/abs/1002.1556"
}

119. Andreas Pedersen, Andrei Manolescu and Ágúst Valfells.
Space-Charge Modulation in Vacuum Microdiodes at THz Frequencies.
Phys. Rev. Lett. 104, 175002 (April 2010).
Abstract We investigate the dynamics of a space-charge limited, photoinjected, electron beam in a microscopic vacuum diode. Because of the small nature of the system it is possible to conduct high-resolution simulations where the number of simulated particles is equal to the number of electrons within the system. In a series of simulations of molecular dynamics type, where electrons are treated as point charges, we address and analyze space-charge effects in a micrometer-scale vacuum diode. We have been able to reproduce breakup of a single pulse injected with a current density beyond the Child-Langmuir limit, and we find that continuous injection of current into the diode gap results in a well-defined train of electron bunches corresponding to THz frequency. A simple analytical explanation of this behavior is given.
URL PDF, DOI BibTeX

@article{PhysRevLett.104.175002,
title = "Space-Charge Modulation in Vacuum Microdiodes at THz Frequencies",
author = "Pedersen, Andreas and Manolescu, Andrei and Valfells, \'Ag\'ust",
journal = "Phys. Rev. Lett.",
volume = 104,
issue = 17,
pages = 175002,
numpages = 4,
year = 2010,
month = "Apr",
doi = "10.1103/PhysRevLett.104.175002",
publisher = "American Physical Society",
abstract = "We investigate the dynamics of a space-charge limited, photoinjected, electron beam in a microscopic vacuum diode. Because of the small nature of the system it is possible to conduct high-resolution simulations where the number of simulated particles is equal to the number of electrons within the system. In a series of simulations of molecular dynamics type, where electrons are treated as point charges, we address and analyze space-charge effects in a micrometer-scale vacuum diode. We have been able to reproduce breakup of a single pulse injected with a current density beyond the Child-Langmuir limit, and we find that continuous injection of current into the diode gap results in a well-defined train of electron bunches corresponding to THz frequency. A simple analytical explanation of this behavior is given.",
}

120. Valeriu Moldoveanu, Andrei Manolescu, Chi-Shung Tang and Vidar Gudmundsson.
Coulomb interaction and transient charging of excited states in open nanosystems.
Phys. Rev. B 81, 155442 (April 2010).
Abstract We obtain and analyze the effect of electron-electron Coulomb interaction on the time-dependent current flowing through a mesoscopic system connected to biased semi-infinite leads. We assume the contact is gradually switched on in time and we calculate the time-dependent reduced density operator of the sample using the generalized master equation. The many-electron states (MES) of the isolated sample are derived with the exact-diagonalization method. The chemical potentials of the two leads create a bias window which determines which MES are relevant to the charging and discharging of the sample and to the currents, during the transient or steady states. We discuss the contribution of the MES with fixed number of electrons N and we find that in the transient regime there are excited states more active than the ground state even for N=1. This is a dynamical signature of the Coulomb-blockade phenomenon. We discuss numerical results for three sample models: short one-dimensional chain, two-dimensional (2D) lattice, and 2D parabolic quantum wire.
URL arXiv, DOI BibTeX

@article{PhysRevB.81.155442,
title = "Coulomb interaction and transient charging of excited states in open nanosystems",
author = "Moldoveanu, Valeriu and Manolescu, Andrei and Tang, Chi-Shung and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 81,
issue = 15,
pages = 155442,
numpages = 12,
year = 2010,
month = "Apr",
doi = "10.1103/PhysRevB.81.155442",
publisher = "American Physical Society",
abstract = "We obtain and analyze the effect of electron-electron Coulomb interaction on the time-dependent current flowing through a mesoscopic system connected to biased semi-infinite leads. We assume the contact is gradually switched on in time and we calculate the time-dependent reduced density operator of the sample using the generalized master equation. The many-electron states (MES) of the isolated sample are derived with the exact-diagonalization method. The chemical potentials of the two leads create a bias window which determines which MES are relevant to the charging and discharging of the sample and to the currents, during the transient or steady states. We discuss the contribution of the MES with fixed number of electrons N and we find that in the transient regime there are excited states more active than the ground state even for N=1. This is a dynamical signature of the Coulomb-blockade phenomenon. We discuss numerical results for three sample models: short one-dimensional chain, two-dimensional (2D) lattice, and 2D parabolic quantum wire.",
arxiv = "http://arxiv.org/abs/1001.0047"
}

121. Vidar Gudmundsson, Cosmin Gainar, Chi-Shung Tang, Valeriu Moldoveanu and Andrei Manolescu.
Time-dependent transport via the generalized master equation through a finite quantum wire with an embedded subsystem.
New Journal of Physics 11, 113007 (2009).
Abstract In this paper, we apply the generalized master equation to analyze time-dependent transport through a finite quantum wire with an embedded subsystem. The parabolic quantum wire and the leads with several subbands are described by a continuous model. We use an approach originally developed for a tight-binding description selecting the relevant states for transport around the bias-window defined around the values of the chemical potential in the left and right leads in order to capture the effects of the nontrivial geometry of the system in the transport. We observe a partial current reflection as a manifestation of a quasi-bound state in an embedded well and the formation of a resonance state between an off-set potential hill and the boundary of the system.
URL arXiv BibTeX

@article{1367-2630-11-11-113007,
author = "Vidar Gudmundsson and Cosmin Gainar and Chi-Shung Tang and Valeriu Moldoveanu and Andrei Manolescu",
title = "Time-dependent transport via the generalized master equation through a finite quantum wire with an embedded subsystem",
journal = "New Journal of Physics",
volume = 11,
number = 11,
pages = 113007,
url = "http://stacks.iop.org/1367-2630/11/i=11/a=113007",
year = 2009,
arxiv = "http://arxiv.org/abs/0903.3491",
abstract = "In this paper, we apply the generalized master equation to analyze time-dependent transport through a finite quantum wire with an embedded subsystem. The parabolic quantum wire and the leads with several subbands are described by a continuous model. We use an approach originally developed for a tight-binding description selecting the relevant states for transport around the bias-window defined around the values of the chemical potential in the left and right leads in order to capture the effects of the nontrivial geometry of the system in the transport. We observe a partial current reflection as a manifestation of a quasi-bound state in an embedded well and the formation of a resonance state between an off-set potential hill and the boundary of the system."
}

122. Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Theoretical investigation of modulated currents in open nanostructures.
Phys. Rev. B 80, 205325 (November 2009).
Abstract We investigate theoretically the transport properties of a mesoscopic system driven by a sequence of rectangular pulses applied at the contact to the input (left) lead. The characteristics of the current which would be measured in the output (right) lead are discussed in relation with the spectral properties of the sample. The time-dependent currents are calculated via a generalized non-Markovian master equation scheme. We study the transient response of a quantum dot and of a narrow quantum wire. We show that the output response depends not only on the lead-sample coupling and on the length of the pulse but also on the states that propagate the input signal. We find that by increasing the bias window the new states available for transport induce additional structure in the relaxation current due to different dynamical tunneling processes. The delay of the output signal with respect to the input current in the case of the narrow quantum wire is associated to the transient time through the wire.
URL PDF, DOI BibTeX

@article{PhysRevB.80.205325,
title = "Theoretical investigation of modulated currents in open nanostructures",
author = "Moldoveanu, Valeriu and Manolescu, Andrei and Gudmundsson, Vidar",
journal = "Phys. Rev. B",
volume = 80,
issue = 20,
pages = 205325,
numpages = 10,
year = 2009,
month = "Nov",
doi = "10.1103/PhysRevB.80.205325",
publisher = "American Physical Society",
abstract = "We investigate theoretically the transport properties of a mesoscopic system driven by a sequence of rectangular pulses applied at the contact to the input (left) lead. The characteristics of the current which would be measured in the output (right) lead are discussed in relation with the spectral properties of the sample. The time-dependent currents are calculated via a generalized non-Markovian master equation scheme. We study the transient response of a quantum dot and of a narrow quantum wire. We show that the output response depends not only on the lead-sample coupling and on the length of the pulse but also on the states that propagate the input signal. We find that by increasing the bias window the new states available for transport induce additional structure in the relaxation current due to different dynamical tunneling processes. The delay of the output signal with respect to the input current in the case of the narrow quantum wire is associated to the transient time through the wire.",
pdf = "http://electronicsandbooks.com/eab1/manual/Magazine/P/Physical%20Review%20B/2009%20Volume%2080/20/PhysRevB.80.205325.pdf"
}

123. Valeriu Moldoveanu, Andrei Manolescu and Vidar Gudmundsson.
Geometrical effects and signal delay in time-dependent transport at the nanoscale.
New Journal of Physics 11, 073019 (2009).
Abstract Nonstationary and steady-state transport through a mesoscopic sample connected to particle reservoirs via time-dependent barriers is investigated by the reduced density operator method. The generalized master equation is solved via the Crank–Nicolson algorithm by taking into account the memory kernel which embodies the non-Markovian effects that are commonly disregarded. The lead–sample coupling takes into account the match between the energy of the incident electrons and the levels of the isolated sample, as well as their overlap at the contacts. Using a tight-binding description of the system, we investigate the effects induced in the transient current by the spectral structure of the sample and by the localization properties of its eigenfunctions. In strong magnetic fields, the transient currents propagate along edge states. The behavior of populations and coherences is discussed, as well as their connection to the tunneling processes that are relevant for transport.
URL arXiv BibTeX

@article{1367-2630-11-7-073019,
author = "Valeriu Moldoveanu and Andrei Manolescu and Vidar Gudmundsson",
title = "Geometrical effects and signal delay in time-dependent transport at the nanoscale",
journal = "New Journal of Physics",
volume = 11,
number = 7,
pages = 073019,
url = "http://stacks.iop.org/1367-2630/11/i=7/a=073019",
year = 2009,
arxiv = "http://arxiv.org/abs/0807.4015",
abstract = "Nonstationary and steady-state transport through a mesoscopic sample connected to particle reservoirs via time-dependent barriers is investigated by the reduced density operator method. The generalized master equation is solved via the Crank–Nicolson algorithm by taking into account the memory kernel which embodies the non-Markovian effects that are commonly disregarded. The lead–sample coupling takes into account the match between the energy of the incident electrons and the levels of the isolated sample, as well as their overlap at the contacts. Using a tight-binding description of the system, we investigate the effects induced in the transient current by the spectral structure of the sample and by the localization properties of its eigenfunctions. In strong magnetic fields, the transient currents propagate along edge states. The behavior of populations and coherences is discussed, as well as their connection to the tunneling processes that are relevant for transport."
}