National Institute Of Materials Physics - Romania

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.

We calculate the orbital magnetization of single and double quantum dots coupled both by Coulomb interaction and by electron tunneling. The electronic states of the quantum dots are calculated in a tight-binding model, and the magnetization is discussed in relation to the energy spectrum and to the edge and bulk states. We identify effects of chirality of the electronic orbits and of the anticrossing of the energy levels when the magnetic field is varied. We also consider the effects of detuning the energy spectra of the quantum dots by an external gate potential. We compare our results with the recent experiments of Oosterkamp [Phys. Rev. Lett. 80, 4951 (1998)].

We investigate the transport properties of quantum dots placed in a strong magnetic field using a quantum-mechanical approach based on the two-dimensional tight-binding Hamiltonian with direct Coulomb interaction and the Landauer-Buttiker formalism. The electronic transmittance and the Hall resistance show Coulomb oscillations and also prove multiple addition processes. We identify this feature as the "bunching" of electrons observed in recent experiments and give an elementary explanation in terms of spectral characteristics of the dot. The spatial distribution of the added electrons may distinguish between the edge and bulk states and it has specific features for bunched electrons. The dependence of the charging energy on the number of electrons is discussed for a strong magnetic field. The crossover from the tunneling to quantum Hall regime is analyzed in terms of dot-lead coupling.

Statistics of level spacing and magnetization are studied for the phase diagram of the integer quantum Hall effect in a two-dimensional finite lattice model with Anderson disorder.

We calculate the quantum states corresponding to the drifting and channeled classical orbits in a two-dimensional electron gas (2DEG) with strong magnetic and electric modulations along one spatial direction, x. The channeled states carry high, concentrated currents along the y-axis, and are confined in an effective potential well. The quantum and the classical states are compared. (C) 1998 Elsevier Science B.V. All rights reserved.

The transport properties of a rectangular mesoscopic plaquette in the presence of a perpendicular magnetic field are studied in a tight-binding model with randomly distributed traps. The longitudinal and Hall resistances are calculated in the four-probe Landauer-Buttiker formalism which accounts automatically both for the quantum coherence and the trapping-induced localization. The localized character of eigenvectors and the specific aspect of the density of states at a given magnetic flux are correlated with the behaviour of the mentioned resistances as function of the Fermi energy. The Hall insulator and quantum Hall regimes are evidenced. The magnetic field dependence of the configurational averages of the longitudinal and Hal; resistance is studied in a purely quantum-mechanical approach. Both, negative and positive magnetoresistances are found.

The spectrum and the eigenstates of a finite two-dimensional tight-binding electronic system, with Dirichlet boundary conditions, in a magnetic field and with an external linear potential are studied. The eigenstates show an equipotential character, and may cross the plaquette in the direction perpendicular to the electric field. When leads are added to the plaquette, the channels carrying the current may be shortcut by equipotentials, resulting in additional plateaus situated between the usual integer-quantum-Hall-effect plateaus. This ideals confirmed by a numerical calculation within the four-terminal Landauer-Buttiker approach.