Publications in 2010

1. 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"
}

2. 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"
}

3. 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.",
}

4. 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"
}

5. D M Danielsson, J T Gudmundsson and H G Svavarsson.
Physica Scripta 2010, 014005 (2010).
Abstract Silicon p-n junctions for solar cell applications were prepared by growing thin n-type films on p-type metallurgical-grade silicon (MG-Si) substrate in a liquid solution of arsenic-doped gallium/indium solution. MG-Si has much higher impurity concentrations than traditional solar-grade silicon, but is here being used as a low-cost alternative. The as-grown film showed negligible photovoltaic response. A post-growth exposure of the as-grown p–n junctions to a H 2 /Ar plasma resulted in an active photovoltaic device with V OC of up to several hundred mV. Furthermore, a significant increase in the minority carrier lifetime was observed. The short-circuit current density indicated low efficiency in accordance with the high defect concentration of the impure substrate. An x-ray diffraction study of MG-Si prior to and after film growth revealed textured crystal structure that diminished during film growth, indicating that the film is more homogeneous than the substrate.
URL BibTeX

@article{1402-4896-2010-T141-014005,
author = "D M Danielsson and J T Gudmundsson and H G Svavarsson",
title = "Effect of hydrogenation on minority carrier lifetime in low-grade silicon",
journal = "Physica Scripta",
volume = 2010,
number = "T141",
pages = 014005,
url = "http://stacks.iop.org/1402-4896/2010/i=T141/a=014005",
year = 2010,
abstract = "Silicon p-n junctions for solar cell applications were prepared by growing thin n-type films on p-type metallurgical-grade silicon (MG-Si) substrate in a liquid solution of arsenic-doped gallium/indium solution. MG-Si has much higher impurity concentrations than traditional solar-grade silicon, but is here being used as a low-cost alternative. The as-grown film showed negligible photovoltaic response. A post-growth exposure of the as-grown p–n junctions to a H 2 /Ar plasma resulted in an active photovoltaic device with V OC of up to several hundred mV. Furthermore, a significant increase in the minority carrier lifetime was observed. The short-circuit current density indicated low efficiency in accordance with the high defect concentration of the impure substrate. An x-ray diffraction study of MG-Si prior to and after film growth revealed textured crystal structure that diminished during film growth, indicating that the film is more homogeneous than the substrate."
}

6. Gunnar Thorgilsson and Sigurdur I Erlingsson.
Effects of scattering area shape on spin conductance in a four-terminal spin-Hall setup.
Phys. Rev. B 82, 245308 (December 2010).
Abstract We study spin conductance in a ballistic and quasiballistic two-dimensional electron system with Rashba spin-orbit coupling. The system has a four-terminal geometry with round corners at the connection to the leads. It is found that by going from sharp corners to more round corners in the ballistic system the energy-depended spin conductance goes from being relatively flat to a curve showing a series of minima and maxima. It is also found that when changing the size of the terminal area by modifying the roundness of the terminal corners the maxima and minima in the transverse spin conductance are shifted in energy. This shift is due increased (decreased) energy for smaller (larger) terminal area. These results were also found to be reasonably stable in quasiballistic systems.
URL arXiv, DOI BibTeX

@article{PhysRevB.82.245308,
title = "Effects of scattering area shape on spin conductance in a four-terminal spin-Hall setup",
author = "Thorgilsson, Gunnar and Erlingsson, Sigurdur I.",
journal = "Phys. Rev. B",
volume = 82,
issue = 24,
pages = 245308,
numpages = 7,
year = 2010,
month = "Dec",
doi = "10.1103/PhysRevB.82.245308",
publisher = "American Physical Society",
abstract = "We study spin conductance in a ballistic and quasiballistic two-dimensional electron system with Rashba spin-orbit coupling. The system has a four-terminal geometry with round corners at the connection to the leads. It is found that by going from sharp corners to more round corners in the ballistic system the energy-depended spin conductance goes from being relatively flat to a curve showing a series of minima and maxima. It is also found that when changing the size of the terminal area by modifying the roundness of the terminal corners the maxima and minima in the transverse spin conductance are shifted in energy. This shift is due increased (decreased) energy for smaller (larger) terminal area. These results were also found to be reasonably stable in quasiballistic systems.",
arxiv = "http://arxiv.org/abs/1111.1529"
}

7. Gunnar Thorgilsson and Sigurdur I Erlingsson.
Transport in four-terminal semiconductor nanostructures with Rashba spin–orbit interaction.
Physica Scripta 2010, 014014 (2010).
Abstract We studied spin transport in a four-terminal system with Rashba spin–orbit coupling. Using discretization, we convert the non-equilibrium Green's function equations into matrix equations, which are then solved using the recursive Green's function method. The calculations show that having round edges in the scattering region leads to a more regular spin polarization, indicating that the shape of the scattering region can be used as an additional control for spintronics applications.
URL BibTeX

@article{1402-4896-2010-T141-014014,
author = "Gunnar Thorgilsson and Sigurdur I Erlingsson",
title = "Transport in four-terminal semiconductor nanostructures with Rashba spin–orbit interaction",
journal = "Physica Scripta",
volume = 2010,
number = "T141",
pages = 014014,
url = "http://stacks.iop.org/1402-4896/2010/i=T141/a=014014",
year = 2010,
abstract = "We studied spin transport in a four-terminal system with Rashba spin–orbit coupling. Using discretization, we convert the non-equilibrium Green's function equations into matrix equations, which are then solved using the recursive Green's function method. The calculations show that having round edges in the scattering region leads to a more regular spin polarization, indicating that the shape of the scattering region can be used as an additional control for spintronics applications."
}

8. Sigurdur I Erlingsson, Carlos J Egues and Daniel Loss.
Energy spectra for quantum wires and two-dimensional electron gases in magnetic fields with Rashba and Dresselhaus spin-orbit interactions.
Phys. Rev. B 82, 155456 (October 2010).
Abstract We introduce an analytical approximation scheme to diagonalize parabolically confined two-dimensional (2D) electron systems with both the Rashba and Dresselhaus spin-orbit interactions. The starting point of our perturbative expansion is a zeroth-order Hamiltonian for an electron confined in a quantum wire with an effective spin-orbit induced magnetic field along the wire, obtained by properly rotating the usual spin-orbit Hamiltonian. We find that the spin-orbit-related transverse coupling terms can be recast into two parts W and V, which couple crossing and noncrossing adjacent transverse modes, respectively. Interestingly, the zeroth-order Hamiltonian together with W can be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics. We treat the V coupling by performing a Schrieffer-Wolff transformation. This allows us to obtain an effective Hamiltonian to third order in the coupling strength $k_R$ℓ of V, which can be straightforwardly diagonalized via an additional unitary transformation. We also apply our approach to other types of effective parabolic confinement, e.g., 2D electrons in a perpendicular magnetic field. To demonstrate the usefulness of our approximate eigensolutions, we obtain analytical expressions for the nth Landau-level gn factors in the presence of both Rashba and Dresselhaus couplings. For small values of the bulk g factors, we find that spin-orbit effects cancel out entirely for particular values of the spin-orbit couplings. By solving simple transcendental equations we also obtain the band minima of a Rashba-coupled quantum wire as a function of an external magnetic field. These can be used to describe Shubnikov-de Haas oscillations. This procedure makes it easier to extract the strength of the spin-orbit interaction in these systems via proper fitting of the data.
URL, DOI BibTeX

@article{PhysRevB.82.155456,
title = "Energy spectra for quantum wires and two-dimensional electron gases in magnetic fields with Rashba and Dresselhaus spin-orbit interactions",
author = "Erlingsson, Sigurdur I. and Egues, J. Carlos and Loss, Daniel",
journal = "Phys. Rev. B",
volume = 82,
issue = 15,
pages = 155456,
numpages = 12,
year = 2010,
month = "Oct",
doi = "10.1103/PhysRevB.82.155456",
abstract = "We introduce an analytical approximation scheme to diagonalize parabolically confined two-dimensional (2D) electron systems with both the Rashba and Dresselhaus spin-orbit interactions. The starting point of our perturbative expansion is a zeroth-order Hamiltonian for an electron confined in a quantum wire with an effective spin-orbit induced magnetic field along the wire, obtained by properly rotating the usual spin-orbit Hamiltonian. We find that the spin-orbit-related transverse coupling terms can be recast into two parts W and V, which couple crossing and noncrossing adjacent transverse modes, respectively. Interestingly, the zeroth-order Hamiltonian together with W can be solved exactly, as it maps onto the Jaynes-Cummings model of quantum optics. We treat the V coupling by performing a Schrieffer-Wolff transformation. This allows us to obtain an effective Hamiltonian to third order in the coupling strength $k_R$ℓ of V, which can be straightforwardly diagonalized via an additional unitary transformation. We also apply our approach to other types of effective parabolic confinement, e.g., 2D electrons in a perpendicular magnetic field. To demonstrate the usefulness of our approximate eigensolutions, we obtain analytical expressions for the nth Landau-level gn factors in the presence of both Rashba and Dresselhaus couplings. For small values of the bulk g factors, we find that spin-orbit effects cancel out entirely for particular values of the spin-orbit couplings. By solving simple transcendental equations we also obtain the band minima of a Rashba-coupled quantum wire as a function of an external magnetic field. These can be used to describe Shubnikov-de Haas oscillations. This procedure makes it easier to extract the strength of the spin-orbit interaction in these systems via proper fitting of the data.",