Publications in 2018

1. 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."
}

2. Tudor D Stanescu, Anna Sitek and Andrei Manolescu.
Robust topological phase in proximitized core-shell nanowires coupled to multiple superconductors.
arXiv preprint arXiv:1804.05446 (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 BibTeX

@article{stanescu2018robust,
title = "Robust topological phase in proximitized core-shell nanowires coupled to multiple superconductors",
author = "Stanescu, Tudor D and Sitek, Anna and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1804.05446",
arxiv = "https://arxiv.org/abs/1804.05446",
year = 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."
}

3. 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."
}

4. 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."
}

5. GA Nemnes, Cristina Besleaga, AG Tomulescu, Alexandra Palici, L Pintilie, A Manolescu and Ioana Pintilie.
How measurement protocols influence the dynamic JV characteristics of perovskite solar cells: theory and experiment.
arXiv preprint arXiv:1803.00285 (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-less 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 forward-reverse or reverse-forward bias scans 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.
arXiv BibTeX

@article{nemnes2018measurement,
title = "How measurement protocols influence the dynamic JV characteristics of perovskite solar cells: theory and experiment",
author = "Nemnes, GA and Besleaga, Cristina and Tomulescu, AG and Palici, Alexandra and Pintilie, L and Manolescu, A and Pintilie, Ioana",
journal = "arXiv preprint arXiv:1803.00285",
arxiv = "https://arxiv.org/abs/1803.00285",
year = 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-less 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 forward-reverse or reverse-forward bias scans 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."
}

6. 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.
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{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",
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."
}

7. 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."
}

8. 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
9. 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."
}