# 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. 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."
}

2. Anna Sitek, Mugurel Tolea, Marian Nita, Llorenç Serra, Vidar Gudmundsson and Andrei Manolescu.
In-gap corner states in core-shell polygonal quantum rings.
arXiv preprint arXiv:1607.02107 (2016).
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 uncoupled electrons, but below the energies of side-localized states. We also specify conditions allowing optical excitation of those states.
arXiv BibTeX

@article{sitek2016gap,
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 = "arXiv preprint arXiv:1607.02107",
arxiv = "http://arxiv.org/abs/1607.02107",
year = 2016,
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 uncoupled electrons, but below the energies of side-localized states. We also specify conditions allowing optical excitation of those states."
}

3. 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."
}

4. George Alexandru Nemnes, Camelia Visan, Dragos Victor Anghel and Andrei Manolescu.
Molecular dynamics of halogenated graphene-hexagonal boron nitride nanoribbons.
arXiv preprint arXiv:1606.00725 (2016).
Abstract The hybrid graphene - hexagonal boron nitride (G-hBN) systems offer new routes in the design of nanoscale electronic devices. Using $ıt ab initio$ 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.
arXiv BibTeX

@article{nemnes2016molecular,
title = "Molecular dynamics of halogenated graphene-hexagonal boron nitride nanoribbons",
author = "Nemnes, George Alexandru and Visan, Camelia and Anghel, Dragos Victor and Manolescu, Andrei",
journal = "arXiv preprint arXiv:1606.00725",
arxiv = "http://arxiv.org/abs/1605.08248",
year = 2016,
abstract = "The hybrid graphene - hexagonal boron nitride (G-hBN) systems offer new routes in the design of nanoscale electronic devices. Using ${\it ab initio}$ 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."
}

5. 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."
}

6. 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."
}

7. 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."
}

8. 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."
}

9. 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."
}

10. 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."
}

11. 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."
}

12. 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."
}

13. 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."
}

14. 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."
}

15. 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"
}

16. 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."
}

17. 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."
}

18. 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"
}

19. 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"
}

20. 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."
}

21. 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."
}

22. 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."
}

23. 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."
}

24. 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."
}

25. 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."
}

26. 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"
}

27. 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."
}

28. 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."
}

29. 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."
}

30. 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."
}

31. 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"
}

32. 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"
}

33. 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"
}

34. 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"
}

35. 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"
}

36. 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"
}

37. 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"
}

38. 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"
}

39. 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 (2013).
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 systems 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{,
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 systems 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"
}

40. 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"
}

41. 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"
}

42. 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"
}

43. 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"
}

44. 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"
}

45. 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"
}

46. 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"
}

47. 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"
}

48. 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"
}

49. 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"
}

50. 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"
}

51. 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"
}

52. 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."
}

53. 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"
}

54. 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"
}

55. 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.",
}

56. 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"
}

57. 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."
}

58. 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"
}

59. 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."
}