We have calculated the density of states (DOS) of a two-dimensional electron gas in a perpendicular magnetic field, using a multiple scattering method, in the ultraquantum limit. We have considered doped and disordered 2D systems. The results of the scattering method are compared with direct simulations of disordered samples. Using the DOS, we have studied the metal-insulator transition and the magnetic freeze-out including a comparison with experimental results. (C) 2004 Elsevier B.V. All rights reserved.

Our study presents experimental measurements of the contact and longitudinal voltage drops in Hall bars, as a function of the current amplitude. We are interested in the heating phenomenon which takes place before the breakdown of the quantum Hall effect, i.e. the pre-breakdown regime. Two types of samples has been investigated, at low temperature (4.2 and 1.5 K) and high magnetic field (up to 13 T). The Hall bars have several different widths, and our observations clearly demonstrate that the size of the sample influences the heating phenomenon. By measuring the critical currents of both contact and longitudinal voltages, as a function of the filling factor (around i = 2), we highlight the presence of a high electric field domain near the source contact, which is observable only in samples whose width is smaller than 400 mum. (C) 2004 Elsevier B.V. All rights reserved.

We perform the investigations of the resonant tunneling via impurities embedded in the AlAs barrier of a single GaAs/AlGaAs heterostructure. In the I(V) characteristics measured at 30 mK, the contribution of individual donors is resolved and the fingerprints of phonon assistance in the tunneling process are seen. The latter is confirmed by detailed analysis of the tunneling rates and the modeling of the resonant tunneling contribution to the current. Moreover, fluctuations of the local structure of the DOS (LDOS) and Fermi edge singularities are observed.

We have used silicon impurities in an aluminum arsenide barrier to probe an adjacent two-dimensional electron gas (2DEG). A single impurity acts as a local spectrometer and scans the local density of states of the 2DEG. Magnetotransport experiments have been performed at low temperature with a magnetic field B applied along the direction of the current. Current-voltage characteristics strongly depend on B and reveal the formation of Landau levels (LLs).

We present calculations of the quantum oscillations appearing in the transmission of a mesoscopic GaAs/GaAlAs ring isolated by quantum point contacts. We show that the device acts as an electronic Fabry-Perot spectrometer in the quantum Hall effect regime, and discuss the effect of the coherence length of edge states.

The effect of pressure on electrical transport phenomena has been studied in AlGaN/GaN heterostructures grown on Si substrates. The two-dimensional character of the conducting carriers has been verified with high magnetic field measurements (up to 13 T) at low temperature (T = 1.5 K). The two-dimensional electron gas (2DEG) concentration and mobility were measured as a function of hydrostatic pressure up to 1100 MPa and at different temperatures. The observed pressure changes of the free carrier concentration have been compared with theoretical predictions, taking into account the polarization change in the nitrides and using a fully self-consistent resolution of Schrodinger and Poisson equations.

Our paper is a first step towards tunnelling spectroscopy of individual X-minimum-related shallow donors. We investigated I-V characteristics of submicrometer mesa structures made on GaAs/AlAs/GaAs wafers delta-doped with silicon in the middle of AlAs layer. At 4.2 K and magnetic field up to 6 T we resolved well-separated peaks attributed to resonant tunnelling via individual donors. (C) 2002 Elsevier Science B.V. All rights reserved.