Publications in 2016

  1. Kristinn Torfason, Agust Valfells and Andrei Manolescu.
    Molecular dynamics simulations of field emission from a prolate spheroidal tip.
    Physics of Plasmas 23, 123119 (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.
    URL arXiv, DOI BibTeX

    @article{torfason2016molecular,
    	author = "Kristinn Torfason and Agust Valfells and Andrei Manolescu",
    	title = "Molecular dynamics simulations of field emission from a prolate spheroidal tip",
    	journal = "Physics of Plasmas",
    	volume = 23,
    	number = 12,
    	pages = 123119,
    	year = 2016,
    	doi = "10.1063/1.4972821",
    	url = "http://dx.doi.org/10.1063/1.4972821",
    	eprint = "http://dx.doi.org/10.1063/1.4972821",
    	arxiv = "http://arxiv.org/abs/1608.06789",
    	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."
    }
    
  2. 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.
    arXiv preprint arXiv:1611.09453 (2016).
    Abstract We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots 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. 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.
    arXiv BibTeX

    @article{gudmundsson2016time,
    	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 = "arXiv preprint arXiv:1611.09453",
    	arxiv = "https://arxiv.org/abs/1611.09453",
    	year = 2016,
    	abstract = "We analyze theoretically the charging current into, and the transport current through, a nanoscale two-dimensional electron system with two parallel quantum dots 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. 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."
    }
    
  3. Cristina Besleaga, Laura Elena Abramiuc, Viorica Stancu, Andrei Gabriel Tomulescu, Marian Sima, Liliana Trinca, Neculai Plugaru, Lucian Pintilie, George Alexandru Nemnes, Mihaiela Iliescu, Halldor Gudfinnur Svavarsson, Andrei Manolescu and Ioana Pintilie.
    Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes.
    The Journal of Physical Chemistry Letters 7, 5168-5175 (2016).
    Abstract We monitored the evolution in time of pinhole-free structures based on FTO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I– from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I– negative space charge accumulated at the perovskite–OMeTAD interface.
    URL, DOI BibTeX

    @article{doi:10.1021/acs.jpclett.6b02375,
    	author = "Besleaga, Cristina and Abramiuc, Laura Elena and Stancu, Viorica and Tomulescu, Andrei Gabriel and Sima, Marian and Trinca, Liliana and Plugaru, Neculai and Pintilie, Lucian and Nemnes, George Alexandru and Iliescu, Mihaiela and Svavarsson, Halldor Gudfinnur and Manolescu, Andrei and Pintilie, Ioana",
    	title = "Iodine Migration and Degradation of Perovskite Solar Cells Enhanced by Metallic Electrodes",
    	journal = "The Journal of Physical Chemistry Letters",
    	volume = 7,
    	number = 24,
    	pages = "5168-5175",
    	year = 2016,
    	doi = "10.1021/acs.jpclett.6b02375",
    	note = "PMID: 27973891",
    	url = "http://dx.doi.org/10.1021/acs.jpclett.6b02375",
    	eprint = "http://dx.doi.org/10.1021/acs.jpclett.6b02375",
    	abstract = "We monitored the evolution in time of pinhole-free structures based on FTO/TiO2/CH3NH3PbI2.6Cl0.4 layers, with and without spiro-OMeTAD and counter electrodes (Ag, Mo/Ag, and Au), aged at 24 °C in a dark nitrogen atmosphere. In the absence of electrodes, no degradation occurs. While devices with Au show only a 10% drop in power conversion efficiency, remaining stable after a further overheating at 70 °C, >90% is lost when using Ag, with the process being slower for Mo/Ag. We demonstrate that iodine is dislocated by the electric field between the electrodes, and this is an intrinsic cause for electromigration of I– from the perovskite until it reaches the anode. The iodine exhaustion in the perovskite layer is produced when using Ag electrodes, and AgI is formed. We hypothesize that in the presence of Au the iodine migration is limited due to the buildup of I– negative space charge accumulated at the perovskite–OMeTAD interface."
    }
    
  4. Thorsteinn H Jonsson, Andrei Manolescu, Hsi-Sheng Goan, Nzar Rauf Abdullah, Anna Sitek, Chi-Shung Tang and Vidar Gudmundsson.
    Efficient determination of the Markovian time-evolution towards a steady-state of a complex open quantum system.
    arXiv preprint arXiv:1610.03223 (2016).
    Abstract Master equations are commonly used to describe time evolution of open systems. We introduce a general method for calculating a Markovian solution of the Nakajima-Zwanzig generalized master equation. We do so for a time dependent transport of interacting electrons through a complex nano scale system in a photon cavity. The central system, described by 120 many-body states in a Fock space, is weakly coupled to the external leads. The very diverse relaxation times of the open system, reflecting radiative or non-radiative transitions, require information about the time evolution through many orders of magnitude. In our approach, the generalized master equation is mapped from a many-body Fock space of states to a Liouville space of transitions. We show that this results in a linear equation which is solved exactly through an eigenvalue analysis, which supplies information on the steady state and the time evolution of the system.
    arXiv BibTeX

    @article{jonsson2016efficient,
    	title = "Efficient determination of the Markovian time-evolution towards a steady-state of a complex open quantum system",
    	author = "Jonsson, Thorsteinn H and Manolescu, Andrei and Goan, Hsi-Sheng and Abdullah, Nzar Rauf and Sitek, Anna and Tang, Chi-Shung and Gudmundsson, Vidar",
    	journal = "arXiv preprint arXiv:1610.03223",
    	arxiv = "https://arxiv.org/abs/1610.03223",
    	year = 2016,
    	abstract = "Master equations are commonly used to describe time evolution of open systems. We introduce a general method for calculating a Markovian solution of the Nakajima-Zwanzig generalized master equation. We do so for a time dependent transport of interacting electrons through a complex nano scale system in a photon cavity. The central system, described by 120 many-body states in a Fock space, is weakly coupled to the external leads. The very diverse relaxation times of the open system, reflecting radiative or non-radiative transitions, require information about the time evolution through many orders of magnitude. In our approach, the generalized master equation is mapped from a many-body Fock space of states to a Liouville space of transitions. We show that this results in a linear equation which is solved exactly through an eigenvalue analysis, which supplies information on the steady state and the time evolution of the system."
    }
    
  5. Joanna M Zajac and Sigurdur I Erlingsson.
    Temperature dependency of resonance fluorescence from InAs/GaAs quantum dots: Dephasing mechanisms.
    Phys. Rev. B 94, 035432 (July 2016).
    Abstract We report a study on temperature-dependent resonant fluorescence from InAs/GaAs quantum dots. We combined spectral and temporal measurements in order to identify sources of dephasing. In the spectral domain, we observed temperature-dependent broadening of the zero-phonon line as 0.3 ?eV/K, and a temperature-dependent phonon broadband. Time-resolved autocorrelation measurements revealed temperature-dependent spin pumping times between T$_1$$_,$$_s$=6 ns (4 K) and 0.5 ns (15 K). These results are compared against theoretical modeling with a master equation for a four-level system coupled to phonon and spin baths. We explained the results by phonon-mediated hole-spin scattering between two excited states, with the piezophonons as a dominant mechanism.
    URL, DOI BibTeX

    @article{PhysRevB.94.035432,
    	title = "Temperature dependency of resonance fluorescence from InAs/GaAs quantum dots: Dephasing mechanisms",
    	author = "Zajac, Joanna M. and Erlingsson, Sigurdur I.",
    	journal = "Phys. Rev. B",
    	volume = 94,
    	issue = 3,
    	pages = 035432,
    	numpages = 7,
    	year = 2016,
    	month = "Jul",
    	publisher = "American Physical Society",
    	doi = "10.1103/PhysRevB.94.035432",
    	url = "http://link.aps.org/doi/10.1103/PhysRevB.94.035432",
    	abstract = "We report a study on temperature-dependent resonant fluorescence from InAs/GaAs quantum dots. We combined spectral and temporal measurements in order to identify sources of dephasing. In the spectral domain, we observed temperature-dependent broadening of the zero-phonon line as 0.3 ?eV/K, and a temperature-dependent phonon broadband. Time-resolved autocorrelation measurements revealed temperature-dependent spin pumping times between T$_1$$_,$$_s$=6 ns (4 K) and 0.5 ns (15 K). These results are compared against theoretical modeling with a master equation for a four-level system coupled to phonon and spin baths. We explained the results by phonon-mediated hole-spin scattering between two excited states, with the piezophonons as a dominant mechanism."
    }
    
  6. 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."
    }
    
  7. R Magnusson, J W Yoon, M Niraula, K J Lee and H G Svavarsson.
    Resonance-based nanophotonic device technology: Filters, reflectors, and absorbers.
    In 2016 IEEE Aerospace Conference. (March 2016), 1-13.
    Abstract We review nanophotonic device technology that is based on fundamental photonic resonance effects. We present the physics behind resonance device operation, illustrate their design with rigorous methods, discuss fabrication processes, and present results of physical and spectral characterization. We indicate the application potential of this field, discuss some past device examples, and provide new and emerging aspects. In particular, we present new wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material thereby eliminating local reflections and phase changes. This critical interface therefore possesses zero refractive-index contrast; hence we call them “zero-contrast gratings.” For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors with nearly 700-nm bandwidth achieved at 99% reflectance. Resonance elements functioning as simultaneous spatial and spectral filters are introduced and substantiated with computed and experimental results that are in excellent agreement. Single-layer bandpass filters are presented and compared to their classic multilayer counterparts. An example bandpass filter with narrow transmission band fashioned with a single periodic layer compares in functionality with a classic Bragg stack with ~30 layers. We discuss deep Si grating structures that efficiently absorb fully-hemispherical unpolarized light in the entire visible spectral domain. This absorber provides a broad spectral continuum of densely populated resonant photonic states as well as a cooperating wide-angular antireflection effect, resulting in broadband, omnidirectional, and polarization-insensitive light absorption. We experimentally verify the absorber performance with precise fabrication and conical input beam spectral analysis. The promise and limitations of this class of devices is discussed.
    DOI BibTeX

    @inproceedings{7500645,
    	author = "R. Magnusson and J. W. Yoon and M. Niraula and K. J. Lee and H. G. Svavarsson",
    	booktitle = "2016 IEEE Aerospace Conference",
    	title = "Resonance-based nanophotonic device technology: Filters, reflectors, and absorbers",
    	year = 2016,
    	pages = "1-13",
    	abstract = "We review nanophotonic device technology that is based on fundamental photonic resonance effects. We present the physics behind resonance device operation, illustrate their design with rigorous methods, discuss fabrication processes, and present results of physical and spectral characterization. We indicate the application potential of this field, discuss some past device examples, and provide new and emerging aspects. In particular, we present new wideband resonant reflectors designed with gratings in which the grating ridges are matched to an identical material thereby eliminating local reflections and phase changes. This critical interface therefore possesses zero refractive-index contrast; hence we call them “zero-contrast gratings.” For simple gratings with two-part periods, we show that zero-contrast grating reflectors outperform comparable high-contrast grating reflectors with nearly 700-nm bandwidth achieved at 99% reflectance. Resonance elements functioning as simultaneous spatial and spectral filters are introduced and substantiated with computed and experimental results that are in excellent agreement. Single-layer bandpass filters are presented and compared to their classic multilayer counterparts. An example bandpass filter with narrow transmission band fashioned with a single periodic layer compares in functionality with a classic Bragg stack with ~30 layers. We discuss deep Si grating structures that efficiently absorb fully-hemispherical unpolarized light in the entire visible spectral domain. This absorber provides a broad spectral continuum of densely populated resonant photonic states as well as a cooperating wide-angular antireflection effect, resulting in broadband, omnidirectional, and polarization-insensitive light absorption. We experimentally verify the absorber performance with precise fabrication and conical input beam spectral analysis. The promise and limitations of this class of devices is discussed.",
    	keywords = "antireflection coatings;band-pass filters;diffraction gratings;elemental semiconductors;light absorption;light polarisation;nanophotonics;optical fabrication;optical filters;periodic structures;silicon;spatial filters;Si;broad spectral continuum;broadband light absorption;conical input beam spectral analysis;critical interface;deep Si grating structures;densely populated resonant photonic states;fully-hemispherical unpolarized light;grating ridges;identical material;narrow transmission band;omnidirectional light absorption;photonic resonance effects;physical characterization;polarization-insensitive light absorption;resonance device operation;resonance elements;resonance-based nanophotonic device technology;single periodic layer;single-layer bandpass filters;spatial filters;spectral characterization;spectral filters;visible spectral domain;wide-angular antireflection effect;wideband resonant reflectors;zero refractive index contrast;zero-contrast grating reflectors;Band-pass filters;Gratings;Nanoscale devices;Reflectivity;Silicon;Wideband",
    	doi = "10.1109/AERO.2016.7500645",
    	month = "March"
    }
    
  8. G A Nemnes, Camelia Visan, D V Anghel and A Manolescu.
    Molecular dynamics of halogenated graphene - hexagonal boron nitride nanoribbons.
    Journal of Physics: Conference Series 738, 012027 (2016).
    Abstract The hybrid graphene - hexagonal boron nitride (G-hBN) systems offer new routes in the design of nanoscale electronic devices. Using ab initio density functional theory calculations we investigate the dynamics of zig-zag nanoribbons a few interatomic distances wide. Several structures are analyzed, namely pristine graphene, hBN and G-hBN systems. By passivating the nanoribbon edges with hydrogen and different halogen atoms, one may tune the electronic and mechanical properties, like the band gap energies and the natural frequencies of vibration.
    URL arXiv, DOI BibTeX

    @article{1742-6596-738-1-012027,
    	author = "G. A. Nemnes and Camelia Visan and D. V. Anghel and A. Manolescu",
    	title = "Molecular dynamics of halogenated graphene - hexagonal boron nitride nanoribbons",
    	journal = "Journal of Physics: Conference Series",
    	volume = 738,
    	number = 1,
    	pages = 012027,
    	url = "http://stacks.iop.org/1742-6596/738/i=1/a=012027",
    	year = 2016,
    	arxiv = "https://arxiv.org/abs/1606.00725",
    	doi = "10.1088/1742-6596/738/1/012027",
    	abstract = "The hybrid graphene - hexagonal boron nitride (G-hBN) systems offer new routes in the design of nanoscale electronic devices. Using ab initio density functional theory calculations we investigate the dynamics of zig-zag nanoribbons a few interatomic distances wide. Several structures are analyzed, namely pristine graphene, hBN and G-hBN systems. By passivating the nanoribbon edges with hydrogen and different halogen atoms, one may tune the electronic and mechanical properties, like the band gap energies and the natural frequencies of vibration."
    }
    
  9. 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,
    	number = "",
    	pages = "197 - 203",
    	year = 2017,
    	note = "",
    	issn = "0927-0248",
    	doi = "http://dx.doi.org/10.1016/j.solmat.2016.09.012",
    	url = "http://www.sciencedirect.com/science/article/pii/S0927024816303531",
    	arxiv = "http://arxiv.org/abs/1606.00335",
    	author = "G.A. Nemnes and Cristina Besleaga and A.G. Tomulescu and Ioana Pintilie and L. Pintilie and K. Torfason and A. Manolescu",
    	keywords = "Dynamic electrical model",
    	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."
    }
    
  10. 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",
    	url = "http://link.aps.org/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."
    }
    
  11. Halldor Gudfinnur Svavarsson, Birgir Hrafn Hallgrimsson, Manoj Niraula, Kyu Jin Lee and Robert Magnusson.
    Large arrays of ultra-high aspect ratio periodic silicon nanowires obtained via top–down route.
    Applied Physics A 122, 1–6 (2016).
    Abstract Metal-catalysed etching (MCE) is a simple and versatile method for fabrication of silicon nanowires, of high structural quality. When combined with laser interference lithography (LIL), large areas of periodic structures can be generated in only few steps. The aspect ratio of such periodic structure is however commonly not higher than several decades or very few hundred. Here, a combined MCE and LIL techniques were applied to fabricate dense (4 \texttimes 108 cm?3), periodic arrays of vertically aligned silicon nanowires with aspect ratio of up to 103. This is a considerable higher number than previously reported on for periodic silicon wire arrays prepared with top–down approaches. The wires were slightly tapered, with top and bottom diameters ranging from 370 to 195 nm and length of up to 200 $\mu$m. A potential use of the nanowires as light absorber is demonstrated by measuring reflection in integrating sphere. An average total absorption of \textasciitilde97 % was observed for 200-$\mu$m-long wires in the spectral range of 450–1000 nm. A comparison to simulated absorption spectra is given.
    URL, DOI BibTeX

    @article{Svavarsson2016,
    	author = "Svavarsson, Halldor Gudfinnur and Hallgrimsson, Birgir Hrafn and Niraula, Manoj and Lee, Kyu Jin and Magnusson, Robert",
    	title = "Large arrays of ultra-high aspect ratio periodic silicon nanowires obtained via top--down route",
    	journal = "Applied Physics A",
    	year = 2016,
    	volume = 122,
    	number = 2,
    	pages = "1--6",
    	abstract = "Metal-catalysed etching (MCE) is a simple and versatile method for fabrication of silicon nanowires, of high structural quality. When combined with laser interference lithography (LIL), large areas of periodic structures can be generated in only few steps. The aspect ratio of such periodic structure is however commonly not higher than several decades or very few hundred. Here, a combined MCE and LIL techniques were applied to fabricate dense (4 {\texttimes} 108 cm?3), periodic arrays of vertically aligned silicon nanowires with aspect ratio of up to 103. This is a considerable higher number than previously reported on for periodic silicon wire arrays prepared with top--down approaches. The wires were slightly tapered, with top and bottom diameters ranging from 370 to 195 nm and length of up to 200 $\mu$m. A potential use of the nanowires as light absorber is demonstrated by measuring reflection in integrating sphere. An average total absorption of {\textasciitilde}97 \% was observed for 200-$\mu$m-long wires in the spectral range of 450--1000 nm. A comparison to simulated absorption spectra is given.",
    	issn = "1432-0630",
    	doi = "10.1007/s00339-015-9589-y",
    	url = "http://dx.doi.org/10.1007/s00339-015-9589-y"
    }
    
  12. 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",
    	url = "http://link.aps.org/doi/10.1103/PhysRevB.93.205445",
    	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."
    }
    
  13. 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."
    }
    
  14. 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",
    	url = "http://link.aps.org/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."
    }
    
  15. 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."
    }
    
  16. 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."
    }
    
  17. 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."
    }
    
  18. 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."
    }