We have investigated experimentally and theoretically the effect of repulsive and attractive ionized impurities on the resistivity components (ρxx and ρxy) in the quantum Hall effect regime. GaAs/GaAlAs asymmetric modulation-doped quantum wells with additional delta doping (by Si donor atoms or Be acceptor atoms) in the GaAs channel or at the AlGaAs/GaAs interface has been grown using molecular beam epitaxy technique. Magnetotransport experiments, performed on samples doped with Si-attractive atoms, showed a plateau width increasing toward lower magnetic field at even filling factor. However, when samples were delta doped with Be repulsive atoms, the increase was observed in the opposite side. Part of the results was explained using a model based on the fifth Klauder’s approximations where we demonstrate that the asymmetrical increase of the Hall plateaus with even filling factor (Landau gaps) is related to the asymmetry induced in the density of states by the additional impurities: the resulting disorder short range potential broadens and shifts the Landau levels but also creates impurity bands on the lower energy side of the Landau levels in the case of donors and on the upper energy side of the Landau levels in the case of acceptors. We notice that this asymmetrical behavior was not experimentally observed for odd filling factor plateaus (exchange gaps). We have also experimentally underscored the screening effect by free two-dimensional electrons of this disorder short range potential. Moreover, for delta-doped Be samples, the whole ν=1 Hall plateau was shifted toward higher magnetic field with respect to the classical Hall effect. This shift, observed for all samples, cannot be explained by the asymmetry of the density of states but rather by a magnetic delocalization of electrons from the upper energy impurity band associated with the last Landau level (n=0) into the free n=0 Landau states when this impurity band overtakes the Fermi level at the end of the ν=2 plateau. This magnetic delocalization effect is the opposite effect of the magnetic freeze out. © 2009 The American Physical Society

We have measured the longitudinal impedance of a two dimensional electron gas in the quantum Hall effect regime. We have found smooth resonances of impedance for the middle of quantized plateaus and large resonances at the beginning and the end of the plateaus. Those resonances can be partially described using a simplified electrical circuit model. The analysis of the different experimental results shows a great influence of the experimental setup on the experimental measurements.

At mesoscopic scale, the variable range hopping can be described by a tunnel effect through a single barrier. We have placed a GaALAs/GaAs HEMT in the quantum Hall regime (high magnetic field and very low temperature) and measured the components of the conductance tensor σxx and σxy. We study the regime of the Shubnikov de Haas peaks and compare it with the same regime in large sample, commonly understood as a VRH regime. We analyse the variation in temperature in the range [0.1K-1K] for kT to scale the fundamental energies, and demonstrate that in the mesoscopic scale, the measured longitudinal conductivity corresponds exactly to the tunneling through a single barrier. We deduce the barrier characteristics by fitting our data with the conductivity of a square tunnel barrier, and we retrieve the characteristic height and length of the bare disorder potential.

We present a low temperature magnetoconductance study of a submicron hall bar made from an AlGaAs/InGaAs/GaAs heterostructure. Experiments were performed at very low temperature (120 mK), with a magnetic field ranging from OT to 13.5 T. In the presence of a quantizing magnetic field, at the transition between two quantized Hall plateaus, a succession of sharp peaks is detected in the Hall signal RH and in the longitudinal resistance R-L. The peaks appearing on the high-v side of the R-L transition appear to be different from the peaks appearing on the low-v side. They mainly differ by their temperature dependence. On the high-v side of the RL transition, the temperature evolution of the peaks is typical for resonant tunneling through a single state in one of the antidots that are progressively formed when the initially occupied LL is emptied. On the low v of the transition, by contrast, the temperature dependence is different. This may be related to the asymmetry of the density of states, or to additional inelastic processes.

We use the resistance fluctuations (RFs) appearing in the integer quantum Hall regime to scan the density of states of a very thin Hall bar. By applying a dc voltage on a top gate, we analyze the correlation properties of the various resistances as a function of the magnetic field and the carrier density. In the gate voltage-magnetic field plane, these RFs follow lines with slopes quantized in unit of filling factor and the slope of these RFs depends on their correlation properties.

Nonlinear dependence of the magnetophotoluminescence (MPL) energies in modulation-doped asymmetric GaAs/Ga0.67Al0.33As quantum wells of different widths are investigated experimentally and theoretically as functions of an external magnetic field. The investigated structures have only one electric subband populated with electrons. Contrary to the theoretical descriptions existing in the literature and based on the oscillations of screening, the observed maxima of MPL energies do not occur at integer filling factors and do not change into minima for higher well widths. We interpret our observations assuming that the oscillations of MPL energies are due to an oscillatory electron transfer between a GaAs well and a reservoir outside the well. We obtain a very good description of the experimental data concerning both the maxima positions and the oscillation amplitudes for different well widths and electron densities. Our interpretation is corroborated by the quantum Hall data obtained on the same samples.