National Institute Of Materials Physics - Romania

We investigate the properties of the electron spin-transmission through an Aharonov-Bohm interferometer with an embedded two-dimensional (2D) multilevel quantum dot containing magnetic impurities. A suitable formalism is developed. The amplitude and the phase of the flip- and non-flip-transmittances are calculated numerically as function of the magnetic field. The effects induced by the exchange interaction and those arising from the 2D character of the dot are shown. (c) 2005 Elsevier B.V. All rights reserved.

We investigate the transport through a quantum ring, a dot, and a barrier embedded in a nanowire in a homogeneous perpendicular magnetic field. To be able to treat scattering potentials of finite extent in a magnetic field we use a mixed momentum-coordinate representation to obtain an integral equation for the multiband scattering matrix. For a large embedded quantum ring we are able to obtain Aharonov-Bohm type of oscillations with superimposed narrow resonances caused by interaction with quasibound states in the ring. We also employ the scattering matrix approach to calculate the conductance through a semiextended barrier or well in the wire. The numerical implementations we resort to in order to describe the cases of weak and intermediate magnetic field allow us to produce high resolution maps of the "near field" scattering wave functions, which are used to shed light on the underlying scattering processes.

We study the dynamics of a one-dimensional Bloch electron subjected to a constant electric field. The periodic potential is supposed to be less singular than the delta-like potential (Dirac comb). We give a rigorous proof of Ao's result that for a large class of initial conditions (high momentum regime) there is no localization in momentum space. The proof is based on the mathematical substantiation of the two simplifying assumptions made in physical literature: the transitions between far away bands can be neglected and the transitions at the quasicrossing can be described by Landau-Zener-type formulas. Using the connection between the above model and the driven quantum ring (DQR) shown by Avron and Nemirovski, our results imply the increase of energy for weakly singular such DQR and appropiate initial conditions. (C) 2005 American Institute of Physics.

A detailed description of the tunneling processes, within Aharonov-Bohm (AB) rings containing two-dimensional quantum dots is presented. We show that the electronic propagation through the interferometer is controlled by the spectral properties of the embedded dots and by their coupling with the ring. The transmittance of the interferometer is computed by the Landauer-Buttiker formula. Numerical results are presented for an AB interferometer containing two coupled dots. The charging diagrams for a double-dot interferometer and the Aharonov-Bohm oscillations are obtained, in agreement with the recent experimental results of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)] We identify conditions in which the system shows Fano line shapes. The direction of the asymetric tail depends on the capacitive coupling and on the magnetic field. We discuss our results in connection with the experiments of Kobayashi et al. [Phys. Rev. Lett. 88, 256806 (2002)] in the case of a single dot.

We study a non-adiabatic excitation of an electron system in a 1D quantum ring radiated by a short terahertz pulse. The response of two models, a continuous and a discrete, is explored. By introducing a spatial asymmetry in the external perturbation, a net current can be generated in the ring at a zero magnetic field. Effect of impurities and ratchets are investigated in combination with symmetric and asymmetric external excitation. (c) 2005 Elsevier B.V. All rights reserved.

We study the transport properties of coherently coupled quantum dots in the quantum Hall regime within the Landauer-Buttiker formalism which captures and explains the experimentally observed features in terms of the spectral properties of the coupled dot system. The subpeak structure of the transmittance spectrum and the charging stability diagrams are obtained and discussed. The role of the intradot and interdot Coulomb interaction are pointed out. We show the subpeak evolution with the magnetic field and predict a specific oscillatory behavior of the Hall resistance in strong magnetic field which can be experimentally tested.

We review some aspects concerning the disorder effects on the spectral properties of 2D electronic systems with relevance for the transport and orbital magnetism in perpendicular magnetic field. The lattice model and random Anderson disorder are used. The discussion is carried out separately for periodic and hard wall boundary conditions. The degree of localization is evidenced by the calculation of the inverse participation number of the eigenstates of the Hamiltonian and a striking asymmetry of the localization effect of states in the Landau band is reported. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

The influence of disorder and interaction effects on the ground state polarization of the two-dimensional correlated electron gas is studied by numerical investigations of the unrestricted Hartree-Fock approach. With the model of Anderson disorder a continuous increase of the spin magnetization until the fully polarized regime is obtained. The ferromagnetic ground state is found to be favorable when the electron number is lowered and the interaction and disorder parameters are suitably chosen.

The charge-density oscillations (plasmons) of a low-density two-dimensional uniform electron gas are studied within the framework of finite temperature and frequency dependent (dynamic) version of Singwi, Tosi, Land, and Sjolander theory and compared with the recent experimental results. The use of the Hartree-Fock approximation for the static structure factor leads to a finite temperature dynamical counterpart of the static Hubbard approximation. We observe important differences between dynamic and static local-field factors as well as between the corresponding plasmon dispersion laws. Our calculated plasmon energies that include dynamic correlations are in very good agreement with the recent experimental results.

Recent Coulomb drag experiments in low-density double-layer electron systems have the power of distinguishing various many-body formulations of the effective interactions. In this work we theoretically study the correlation effects on the drag resistivity in these systems within various models. The effective inter-layer interactions are best described by the generalization to the double-layer case of the Kukkonen-Overhauser approach which differs significantly from the self-consistent field approach of Singwi et al. [Phys. Rev. 176 (1968) 589]. Following the formulation of Vignale and Singwi [Phys. Rev. B 32 (1985) 2156] we derive an expression for the effective inter-layer interaction which embodies the many-body correlations through the local-field corrections. The drag resistivity is calculated within this approach together with the Hubbard approximation for the intra-layer local-field factor and a simple model for the inter-layer correlations. Comparison with the recent measurements of Kellogg et al. [Solid State Commun. 123 (2002) 5151 yields very good agreement. Our results are also contrasted with the corresponding drag resistivities given by the Singwi et al. theory, the dynamic random-phase approximation and the Hubbard approximation. The significant differences found between these theories emphasize the strong sensitivity of the drag resistivity to the effective inter-layer interactions. (C) 2003 Elsevier Science Ltd. All rights reserved.