National Institute Of Materials Physics - Romania

When subjected to a linearly polarized terahertz pulse, a mesoscopic ring endowed with spin-orbit interaction (SOT) of the Rashba-Dresselhaus type exhibits non-uniform azimuthal charge and spin distributions. Both types of SOT couplings are considered linear in the electron momentum. Our results are obtained within a formalism based on the equation of motion satisfied by the density operator which is solved numerically for different values of the angle 0, the angle determining the polarization direction of the laser pulse. Solutions thus obtained are later employed in determining the time-dependent charge and spin currents, whose values are calculated in the stationary limit. Both these currents exhibit an oscillatory behavior complicated in the case of the spin current by a beating pattern. We explain this occurrence on account of the two spin-orbit interactions which force the electron spin to oscillate between the two spin quantization axes corresponding to Rashba and Dresselhaus interactions. The oscillation frequencies are explained using the single particle spectrum.

We study the conductance G and Seebeck coefficient S of a side-coupled double quantum dot system. The transport properties are the result of the competition between the Fano interference and Kondo correlations, being also controlled by the Coulomb blockade of the multilevel side-dot. The external parameters are the gate potential V-g applied on the side-dot and the temperature. When V-g is varied continuously the blockade is switched on and off periodically, resulting in oscillations of the transport coefficients. The profile of the oscillations and the temperature dependence are studied. An extended Anderson model and the Keldysh transport formalism are used.

The zero bias anomaly of the differential conductance of mesoscopic systems is a fingerprint of the electron-electron interaction. The common example is the peak of the differential conductance in Kondo mesoscopic systems. We show that in complex mesoscopic systems, in particular in the side-coupled double dot, the dip of the differential conductance at zero bias is also possible due to simultaneous effects of interference and correlations. The external parameters that control this effect are the temperature and the gate potential on the lateral dot. We argue that even in single dots, in a multiple lead configuration, the shift from suppression to enhancement of dI/dV with increasing bias is allowed if the bias applied on the leads is not symmetric. The differential conductance exhibits a peak-dip crossover, the effect being controlled by the strength of the asymmetry and the ratio of the dot-lead couplings. The scaling of the differential conductance for the double-dot system is also discussed. The results are obtained using an extended Anderson model, the Keldysh transport formalism and the equation of motion technique extended to the non-equilibrium.

Under certain conditions, we prove the existence of a steady-state transport regime for interacting mesoscopic systems coupled to reservoirs (leads). The partitioning and partition-free scenarios are treated on an equal footing. Our time-dependent scattering approach is exact and proves, among other things, the independence of the steady-state quantities from the initial state of the sample. Closed formulas for the steady-state current amenable for perturbative calculations with regard to the interaction strength are also derived. In the partitioning case, we calculate the first-order correction and recover the mean-field (Hartree-Fock) results.

Consider a bunch of interacting electrons confined in a quantum dot. The later is suddenly coupled to semi-infinite biased leads at an initial instant t = 0. We identify the dominant contribution to the ergodic current in the off-resonant transport regime, in which the discrete spectrum of the quantum dot is well separated from the absolutely continuous spectrum of the leads. Our approach allows for arbitrary strength of the electron-electron interaction while the current is expanded in even powers of the (weak) lead-dot hopping constant tau. We provide explicit calculations for sequential tunneling and cotunneling contributions to the current. In the interacting case, it turns out that the cotunneling current depends on the initial many-body configuration of the sample, while in the non-interacting case, it does not and coincides with the first term in the expansion of the Landauer formula w.r.t. tau.

We investigate theoretically the transport properties of the side-coupled double quantum dots in connection with the experimental study of Sasaki et al. [Phys. Rev. Lett. 103, 266806 (2009)]. The setup consists of connecting the Kondo dot directly to the leads, while the side dot provides an interference path which affects the Kondo correlations. We analyze the oscillations of the source-drain current due to the periodical Coulomb blockade of the many-level side dot at the variation of the gate potential applied on it. The Fano profile of these oscillations may be controlled by the interdot coupling, level spacing of the side dot, and also by the temperature. The nonequilibrium conductance of the double-dot system exhibits zero-bias anomaly which, besides the usual enhancement, may show also a suppression (a diplike aspect) which occurs around the Fano zero. In the same region, the weak temperature dependence of the conductance indicates the suppression of the Kondo effect. Scaling properties of the nonequilibrium conductance in the Fano-Kondo regime are discussed. Since the Kondo temperature of the single-impurity Anderson model is no longer the proper scaling parameter, we look for an alternative specific to the double dot. The extended Anderson model, Keldysh formalism, and equation-of-motion technique are used.

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.

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.

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

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.