National Institute Of Materials Physics - Romania

We present studies of the role of charge fluctuations in transport through a quantum point contact (QPC). Our model of QPC is based on assumption of a specific electronic energy structure with a resonant level below the electronic sub-band. We show that charge fluctuations lead to a dynamical Coulomb blockade effect, which is responsible for reduction of the conductance to the value 2/3 x (2e(2)/h). The evolution of conductance with a magnetic field and as a function of source-drain voltage is presented as well. The conductance plateau at 2/3 x (2e(2)/h) evolves continuously with an increase of the voltage to a 0.8 or 0.4 plateau, depending on the relative position of the resonant state with respect to the Fermi energy at zero bias. Our simple model shows many similarities with experimental characteristics. (C) 2007 Elsevier B.V. All rights reserved.

The effect of electron-electron interactions on coherent transport in quantum dot systems is theoretically investigated by adapting the well-known random-phase approximation (RPA) to the nonequilibrium GreenKeldysh formalism for open mesoscopic systems. The contour-ordered polarization operator is computed in terms of the Green functions of the noninteracting system. We apply the proposed RPA-Keldysh scheme for studying Coulomb-modified Fano lines and dephasing effects in interferometers with side-coupled many-level dots. Our method allows us to treat on equal footing the decoherence induced by the intradot interaction and that by the Coulomb coupling to a nearby system. In the case of a single interferometer, we show that the intradot Coulomb interaction leads to a reduction of-the Fano line amplitude. From the analysis of the interaction self-energy, it follows that this effect originates in inelastic scattering processes in which electron-hole pairs are involved. The interplay between the interdot and the intradot interactions in decoherence is discussed for two nearby identical T-shaped interferometers. We also show that the intradot interaction does not prevent the observation of controlled dephasing due to a nearby charge detector, as long as the latter is subjected to a sufficiently large bias.

We investigate the measurement induced dephasing of the Fano effect in the electronic transport through a double quantum dot mesoscopic interferometer coupled to a charge detector. The current and the differential conductance are computed within the Keldysh formalism, taking into account of the inelastic processes due to the dot-detector interaction. We show that the visibility of the Fano lineshape is reduced by applying a finite bias on the charge detector. (C) 2007 Elsevier B.V. All rights reserved.

We consider an ideal parabolic quantum wire in a perpendicular magnetic field. A simple Gaussian-shaped scattering potential well or hill is flashed softly on and off with its maximum at t=0, mimicking a temporary broadening or narrowing of the wire. By an extension of the Lippmann-Schwinger formalism to time-dependent scattering potentials, we investigate the effects on the continuous current that is driven through the quantum wire with a vanishingly small forward bias. The Lippmann-Schwinger approach to the scattering process enables us to investigate the interplay between geometrical effects and effects caused by the magnetic field.

The paper presents a few examples relevant for the application of radiofrequency plasmas for processing of materials and surfaces. The examples have been chosen in order to match the trends in plasma processing science, namely processing at atmospheric pressure, growth of nanostructured materials and design of filtration systems responsive to external stimuli.

We present a theoretical study of the electronic transport through a many-level quantum dot driven by time-dependent signals applied at the contacts to the leads. If the barriers oscillate out of phase, the system operates like a turnstile pump under a finite constant bias, as observed in the experiments of Kouwenhoven [Phys. Rev. Lett. 67, 1626 (1991)]. The time-dependent currents and their averages over successive pumping periods are computed from the Keldysh formalism for tight-binding models. The calculation considers a sudden application of the pumping potentials at t=0, which leads to transient features of the time-dependent and averaged currents during the first pumping cycles which turn out to be important in the high-frequency regime. We show that in the transient regime, the efficiency of the system as a pump is rather poor because it mainly absorbs charge from both leads in order to fill the levels located below the bias window. Under a finite bias and a low-frequency pumping signal, the charge transferred across the system depends on the number of levels located within the bias window. The internal charge dynamics and the role of energy sidebands are investigated. The so-called satellite peaks of the averaged current are also observed in the transient regime.

We apply the Keldysh formalism in order to derive a current formula easy to use for a system with many sites, one of which is interacting. The main technical challenge is to deal with the lesser Green's function. It turns out that, in the case of the left-right symmetry, the knowledge of the lesser Green's function is not necessary and an exact current formula can be expressed in terms of retarded Green's functions only. The application is done for a triangular interferometer which gives a good account of the Fano-Kondo effect. It is found that the interference effects, in the context of Kondo correlations, give rise to a point in the parameter space where the conductance is temperature independent. We include a comparison with the results from Ng's ansatz [Phys. Rev. Lett. 70, 3635 (1993)], which are less accurate, but can be used also in the absence of the above mentioned symmetry.

We investigate nonstationary electronic transport in noninteracting nanostructures driven by a finite bias and time-dependent signals applied at their contacts to the leads. The systems are modeled by a tight-binding Hamiltonian, and the transient currents are computed from the nonequilibrium Green-Keldysh formalism. The numerical implementation is not restricted to weak coupling to the leads and does not imply the wideband limit assumption for the spectral width of the leads. As an application of the method we study in detail the transient behavior and the charge dynamics in single and double quantum dots connected to leads by a steplike potential, but the method allows as well consideration of nonperiodic potentials or short pulses. We show that when the higher-energy levels of the isolated system are located within the bias window of the leads, the transient current approaches the steady state in a nonoscillatory smooth fashion. At moderate coupling to the leads and fixed bias the transient acquires a steplike structure, the length of the steps increasing with system size. The number of levels inside a finite bias window can be tuned by a constant gate potential. We find also that the transient behavior depends on the specific way of coupling the leads to the mesoscopic system.

A problem of electronic correlations is considered for two specific mesoscopic systems: the quantum point contact (QPC) and the double quantum dot (2QD) system. The systems are described using a generalized Anderson Hamiltonian. We show that charge fluctuations are relevant for electronic transport. In the QPC a local accumulation of charge and the dynamical Coulomb blockade effect lead to the 0.7 structure in the conductance characteristics. The evolution of the conductance with a magnetic field and in non-equilibrium situations is presented as well. The double quantum dot is studied in the approach, in which correlations within the 2QD are treated exactly, whereas the coupling of the 2QD to the leads is considered in the approximation valid at temperatures above the Kondo temperature. We analyse the evolution of the gate voltage dependence of the spin correlation functions and the conductance with the change of the interdot hopping. For the hopping parameter greater than a threshold value of the on-dot repulsion the physics of the device is dominated by the ground state eigenstates of the 2QD and antiferromagnetic correlations in the case of the doubly occupied 2QD. With a decrease of the interdot hopping repulsion below the threshold we observe a significant reduction of the antiferromagnetic coupling between the dots together with an enhanced occupation of the triplet states.

We show that, due to band mixing, the eigenstate localization within the disordered Landau bands gets an asymmetric structure: the degree of localization increases in the lower part of the band and decreases in the upper one. The calculation is performed for a two-dimensional lattice with the Anderson disorder potential and we prove that this effect is related to the upper shift of the extended states within the band and is enhanced by the disorder strength. The asymmetric localization and the energy shift disappear when the interband coupling is switched off.