National Institute Of Materials Physics - Romania

Publications

141 articles found

81. Nonadiabatic generation of a pure spin current in a one-dimensional quantum ring with spin-orbit interaction

Authors: Nita, M; Marinescu, DC; Manolescu, A; Gudmundsson, V

Published: APR 18 2011, PHYSICAL REVIEW B, 83, DOI: 10.1103/PhysRevB.83.155427

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 phi. 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 phi at which the charge current disappears, while the spin current reaches a maximum or a minimum value.

82. Turnstile pumping through an open quantum wire

Authors: Gainar, CM; Moldoveanu, V; Manolescu, A; Gudmundsson, V

Published: JAN 2011, NEW JOURNAL OF PHYSICS, 13, DOI: 10.1088/1367-2630/13/1/013014

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.

83. Time-dependent magnetotransport in semiconductor nanostructures via the generalized master equation

Authors: Gudmundsson, V; Tang, CS; Gainar, CM; Moldoveanu, V; Manolescu, A

Published: JAN 2011, COMPUTER PHYSICS COMMUNICATIONS, 182, 48, DOI: 10.1016/j.cpc.2010.08.006

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 (C) 2010 Elsevier BV All rights reserved

84. Current enhancement and negative differential conductance in parallel quantum dot systems

Authors: Tanatar, B; Moldoveanu, V

Published: 2011, PHYSICS OF SEMICONDUCTORS: 30TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS, 1399, DOI: 10.1063/1.3666366

We present calculations on the transport properties of a double quantum dot (DQD) capacitively coupled to another individually biased dot. The effects of the intradot and interdot Coulomb interaction are included within the random-phase approximation (RPA) implemented in the Keldysh formalism. We show that by increasing the bias on the nearby dot the inelastic Coulomb scattering modifies the current in the double dot. The sign of the current depends on the detuning of the double dot levels and intradot transitions lead to negative differential conductance. The enhancement of the current due to the energy quanta transferred from the strongly biased dot suggests a quantum ratchet or Coulomb drag mechanism.

85. Spin splitter regime of a mesoscopic Rashba ring

Authors: Moldoveanu, V; Tanatar, B

Published: DEC 1 2010, PHYSICS LETTERS A, 375, 191, DOI: 10.1016/j.physleta.2010.10.047

Using the non-equilibrium Greens' function formalism we calculate the spin currents in a one-dimensional ring coupled to three leads and in the presence of perpendicular magnetic flux (1) and Rashba spin-orbit coupling. A finite bias is applied between the input lead and the other two output leads. We show that the spin-orbit coupling allows one to operate this system as a spin splitter, i.e. the output leads deliver spin-polarized currents with different orientations. We find that the spin splitter operation can be tuned at integer multiples of phi/phi(0). Its efficiency depends not only on the value of the Rashba coupling but also on the bias applied between the input and output leads. The selected spin orientation of the output leads can be reversed by a slight change of their contact position. We discuss as well the connection between the spin splitter operation and the spectral properties of the ring. (C) 2010 Elsevier B.V. All rights reserved.

86. Inelastic transitions and counterflow tunneling in double-dot quantum ratchets

Authors: Moldoveanu, V; Tanatar, B

Published: NOV 10 2010, PHYSICAL REVIEW B, 82, DOI: 10.1103/PhysRevB.82.205312

The ratchet regime of unbiased double quantum dots driven out of equilibrium by an independently biased nearby detector has been theoretically studied using the nonequilibrium Keldysh formalism and the random-phase approximation for the Coulomb effects. When the detector is suitably biased the energy exchange between the two systems removes the Coulomb blockade on the double dot via inelastic interdot tunneling. The energy detuning determines whether the current flows in the same direction as the driving current (positive flow) or in the opposite direction (electronic counterflow). In both cases the intradot transitions lead to negative-differential conductance. Besides the ratchet contribution to the current we also single out a Coulomb drag component.

87. Dynamic correlations induced by Coulomb interactions in coupled quantum dots

Authors: Moldoveanu, V; Manolescu, A; Gudmundsson, V

Published: AUG 11 2010, PHYSICAL REVIEW B, 82, DOI: 10.1103/PhysRevB.82.085311

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.

88. Analyzing the measured phase in the multichannel Aharonov-Bohm interferometer

Authors: Tolea, M; Nita, M; Aldea, A

Published: JUL 2010, PHYSICA E-LOW-DIMENSIONAL SYSTEMS & NANOSTRUCTURES, 42, 2236, DOI: 10.1016/j.physe.2010.04.022

We address the quantum dot phase measurement problem in an open Aharonov-Bohm interferometer, assuming multiple transport channels. In such a case, the quantum dot is characterized by more than one intrinsic phase for the electrons transmission. It is shown that the phase which would be extracted by the usual experimental method (i.e. by monitoring the shift of the Aharonov-Bohm oscillations, as in Schuster et al., Nature 385 (1997) 417) does not coincide with any of the dot intrinsic phases, but is a combination of them. The formula of the measured phase is given. The particular case of a quantum dot containing a S = 1/2 spin is discussed and variations of the measured phase with

89. Correlated time-dependent transport through a two-dimensional quantum structure

Authors: Gudmundsson, V; Tang, CS; Jonasson, O; Moldoveanu, V; Manolescu, A

Published: MAY 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.205319

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.

90. Coulomb interaction and transient charging of excited states in open nanosystems

Authors: Moldoveanu, V; Manolescu, A; Tang, CS; Gudmundsson, V

Published: APR 15 2010, PHYSICAL REVIEW B, 81, DOI: 10.1103/PhysRevB.81.155442

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.