National Institute Of Materials Physics - Romania

An accurate theoretical treatment of electron-electron interactions in mesoscopic systems is available in very few cases and approximation schemes are developed in most of the applications, especially for many-level quantum dots. Here we present transport calculations within the random-phase approximation for the Coulomb interaction using the Keldysh Green's functions formalism. We describe the quantum dot systems by a tight-binding Hamiltonian. Our method is similar to the one used by Faleev and Stockman [Phys. Rev. B 66 085318 (2002)] in their study of the equilibrium properties of a homogeneous 2D electron gas. The important extension at the formal level is that we combine the RPA and the Keldysh formalism for studying non-linear transport properties of open quantum dots. Within the Keldysh formalism the polarization operator becomes a contour-ordered quantity that should be computed either from the non-interacting Green functions of the coupled quantum dot (the so-called G(0)W approximation) either self-consistently (GW approximation). We performed both non-selfconsistent and self-consistent calculations and compare the results. In particular we recover the Coulomb diamonds for interacting quantum dots and we discuss the charge sensing effects in parallel quantum dots.

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.

We study theoretically the electronic transport in parallel few-level quantum dots in the presence of both intradot and interdot long-range Coulomb interaction. Each dot is connected to two leads and the steady-state currents are calculated within the Keldysh formalism using the random-phase approximation for the interacting Green functions. Due to the momentum transfer between the two systems it is possible to get a nonvanishing current through an unbiased Coulomb-blockaded dot, if the other dot is set in the nonlinear transport regime. The transitions between the levels of the passive dot reduce the drag current and lead to negative differential conductance. We also discuss the dependence on temperature and the role of the lead-dot coupling. Copyright (C) EPLA, 2009

We study the differential conductance in the Kondo regime of a quantum dot coupled to multiple leads. When the bias is applied symmetrically on two of the leads (V and -V, as usual in experiments) while the others are grounded, the conductance through the biased leads always shows the expected enhancement at zero bias. However, under asymmetrically applied bias (V and lambda V, with lambda>0), a suppression-dip-appears in the differential conductance if the asymmetry coefficient lambda is beyond a given threshold lambda(0)=3 root 1+r determined by the ratio r of the dot-lead couplings. This is a recipe to determine experimentally this ratio which is important for the quantum-dot devices. This finding is a direct result of the Keldysh transport formalism. For the illustration we use a many-lead Anderson Hamiltonian, the Green's functions being calculated in the Lacroix approximation, which is generalized to the case of nonequilibrium.

We study the coherent transport in a one-dimensional lead with two side-coupled quantum dots using the Keldysh's Green function formalism.The effect of the interdot Coulomb interaction is taken into account by computing the firstand second order contributions to the self-energy.We show that the Fano interference due to the resonance of one dotis strongly affected by the fixed parameters that characterize the second dot. If the second dot is tuned close to resonance an additionalpeak develops between the peak and dip of the Fano line shape of the current. In contrast, when the second dotis off-resonance and its occupation number is close to unity the interdot Coulomb interaction merely shifts the Fano line and no other maxima appear.The system we consider is more general than the single-dot interferometer studied experimentally by Kobayashi et al. [Phys. Rev. B 70, 035319 (2004)] and may be used for controlling quantum interference and studying decoherence effects in mesoscopic transport.

We investigate the differential conductance dI/dV for the interacting T-shape model, using an approach based on the Keldysh formalism and the Lacroix solution for the equation of motion. A peak-dip crossover has been noticed by changing the hybridization between the two dots. For the same model, the combined interaction and interference processes give rise to the Fano-Kondo effect with an interesting crossing point of the isoterms below the Kondo temperature A tentative explanation of these effects is given in terms of the many-body spectral properties of the system.

We study theoretically the electronic transport in parallel few-level quantum dots in the presence of both intradot and interdot long-range Coulomb interaction. Each dot is connected to two leads and the steady-state currents are calculated within the Keldysh formalism using the random-phase approximation for the interacting Green functions. Due to the momentum transfer mechanism between the two systems it is possible to get a nonvanishing current through an unbiased Coulomb-blockaded dot if the other dot is set in the nonlinear transport regime. The transitions between the levels of the passive dot reduce the drag current and lead to negative differential conductance.

We introduce a theoretical formalism describing a wide class of 'Which Path' experiments in mesoscopic/nanoscopic transport. The physical system involves a mesoscopic interferometer (e.g. an Aharonov-Bohm ring with embedded dots or a side-coupled quantum dot) which is electrostatically coupled to a nearby quantum point constriction. Due to the charge sensing effect the latter acts as a charge detector. Therefore the interference pattern can be monitored indirectly by looking at the current characteristics of the detector as shown in the experimental work of Buks et al. [E. Buks, R. Schuster, M. Heiblum, D. Mahalu, V. Umansky, Nature (London) 391 (1998) 871]. We use the non-equilibrium Green-Keldysh formalism and a second order perturbative treatment of the Coulomb interaction in order to compute the relevant transport properties. it is shown that in the presence of the Coulomb interaction the current through the detector exhibits oscillations as a function of the magnetic field applied on a single-dot AB interferometer. We also discuss the dependence of the visibility of the Aharonov-Bohm oscillations on the gate potential applied to the dot. (C) 2008 Elsevier B.V. All rights reserved.

Polymer-like organic thin films obtained by plasma techniques are of wide interest for various applications. Polythiophene-like thin films were deposited by plasma polymerization in a parallel plate radiofrequency reactor starting from thiophene vapors. The optical properties of the films were obtained from spectroscopic ellipsometry, in the 250-1250 nm range. The calculated band gap is in agreement with a polythiophene-like material consisting of cross-linking of oligomers. The presence of functional groups and film stability against oxidation is analyzed by FTIR spectrometry. The monitoring of functional groups incorporation during the film deposition was done by analyzing the integral intensities of specific absorption bands and led to the conclusion that homogeneous films are obtained whatever the process duration.

We analyze the electronic transport through a quantum dot that contains a magnetic impurity. The coherent transport of electrons is governed by the quantum confinement inside the dot, but is also influenced by the exchange interaction with the impurity. The interplay between the two gives raise to the singlet-triplet splitting of the energy levels available for the tunneling electron. In this paper, we focus on the charge fluctuations and, more precisely, the height of the conductance peaks. We show that the conductance peaks corresponding to the triplet levels are three times higher than those corresponding to singlet levels, if electronic correlations are neglected (for non-interacting dots, when an exact solution can be obtained). Next, we consider the Coulomb repulsion and the many-body correlations. In this case, the singlet/triplet peak height ratio has a complex behavior. Usually the highest peak corresponds to the state that is lowest in energy (ground state), regardless if it is singlet or triplet. In the end, we get an insight on the Kondo regime for such a system, and show the formation of three Kondo peaks. We use the equation of motion method with appropriate decoupling.