M. Goiran

Cyclotron resonance and quantum Hall effect studies of the two-dimensional electron gas confined at the GaN/AlGaN interface

We report on high magnetic fields (up to 40 T) cyclotron resonance, quantum Hall effect and Shubnikov-de-Hass measurements in high frequency transistors based on Si-doped GaN–AlGaN heterojunctions. A simple way of precise modelling of the cyclotron absorption in these heterojunctions is presented. We clearly establish two-dimensional electrons to be the dominant conducting carriers and determine precisely their in-plane effective mass to be 0.230±0.005 of the free electron effective mass. The increase of the effective mass with an increase of two-dimensional carrier density is observed and explained by the nonparabolicity effect.

Atomic scale saw by dislocation slipping: A new method to generate one‐dimensional structure

A new method to generate one‐dimensional (1D) semiconductor structures or controllable steps on a surface is proposed. This method, which is not material specific, is based on intrinsic dislocation slip properties and heterogeneity of straining. Dislocations can be used as an atomic scale saw to cut two‐dimensional structures (2D) in order to obtain 1D wires. Atomic force microscope observations of GaAs surfaces and transmission electronic microscopy cross sections of GaAs/GaAlAs single quantum wells are presented to demonstrate the feasibility of the method. Although being based on dislocation slipping this method is shown to preserve the optical and crystalline properties of the starting 2D structure as confirmed by photoluminescence spectra.

High magnetic field studies of two-dimensional electron gas in a GaN/GaAlN heterostructure: Mechanisms of parallel conduction

We report on quantum Hall effect (QHE) and Shubnikov–de-Haas (SdH) measurements for a GaN/GaAlN heterostructure in high magnetic fields (up to 35 T). The observed well defined SdH oscillations, correlated with QHE plateaus, confirm the existence of a two-dimensional electron gas at the GaN/GaAlN interface. The Shubnikov–de-Haas oscillations are superimposed on a strong positive magnetoresistance. We show that this positive magnetoresistance can be linked to a parallel conduction channel. The analysis of the high magnetic field data yields information about the carrier concentration and mobility in this channel. The concentration and mobility of two-dimensional gas and the parallel conduction layer are analyzed in a wide temperature range (1.9–77 K). A physical model (based on the band diagram and mobility calculations) explaining the origin of parallel conduction is presented.

Spin transitions in a series of FeII molecular complexes induced by a strong-pulsed magnetic field

Abstract We report on the first comparative investigations of Fe II spin-crossover complexes under high-pulsed magnetic fields near the transition temperature T c . A partial irreversible triggering of the spin conversion is evidenced when the magnetic field is applied at a temperature belonging to the heating branch of the thermal hysteresis. A strong correlation between the strength of the cooperative interactions and the kinetic character of the spin conversion process is demonstrated.

High field magneto-transport in two-dimensional electron gas LaAlO3/SrTiO3

The transport properties of the complex oxide LaAlO3/SrTiO3 interface are investigated under a high magnetic field (55u2009T). Small oscillations of the magnetoresistance with altered periodicity are observed when plotted versus the inverse magnetic field. We attribute this effect to Rashba spin-orbit coupling which remains consistent with large negative magnetoresistance when the field is parallel to the sample plane. A large inconsistency between the carrier density extracted from Shubnikov-de Haas analysis and from the Hall effect is explained by the contribution to transport of at least two bands with different mobilities.

Two-level Ising-like model for spin-crossover phenomenon including the magnetic field effect: the mean-field approximation and Monte Carlo resolutions

Abstract In this paper we analysed the magnetic field effect on the spin-crossover phenomenon using different theoretical approaches: thermodynamic and Ising-like models. This latter, including magnetic field, was treated in the mean-field approximation and—for the first time—by Monte Carlo numerical method. We propose an adaptation of the metropolis algorithm to take into account the different degeneracies of the high and low-spin levels and their Zeeman splitings due to the effect of magnetic field in both states. A comparison of both methods is given and discussed.

Fractal analysis of atomic force microscopy pictures of slip lines on a GaAs/GaAlAs heterostructure plastically deformed to obtain quantum wires

A fractal analysis of slip lines on the surface of a plastically deformed GaAs/GaAlAs heterostructure has been performed using atomic force microscopy. The high resolution of atomic force microscopy allowed us to analyze a very large interval of magnifications ranging from 1100 to 1u2009110u2009000. The fractal behavior of the slip line pattern was only observed in a decade of magnification. At the highest magnifications (40u2009000–1u2009110u2009000), the less fractal tendency of the slip line pattern confirms that the slipping of dislocations is an efficient method to cut a quantum well into quantum wires with a ‘‘natural’’ reasonable degree of homogeneity.

Magnetic properties of granular ferromagnetics

Abstract Experiments demonstrate that the magnetization of Fe x (SiO 2 ) 1− x -nanocomposite at magnetic fields H ⩽20xa0T depends rather weakly on temperature T =4–300xa0K. Thus, the Langevin model for the magnetization is invalid. We explain our results in the frame of a simple model taking into account distributions of non-spherical granules’ orientations and forms. Calculated field dependence of the magnetization agrees with the experiment everywhere except for the fields H ∼10xa0kOe, where calculated values are somewhat higher (up to ∼20%) than experimental ones. These discrepancies are related to the magnetic “glass” behavior of the magnetization.

High magnetic field studies of quantum transport and cyclotron resonance on 2D gas in GaN/GaAlN heterojunction

Abstract Magnetotransport and cyclotron resonance measurements were performed on Si-doped GaN/GaAlN heterostructure under pulsed magnetic field up to 35xa0T. The existence of 2D gas in the structure has been confirmed by cyclotron absorption and Shubnikov–de-Haas oscillations measurements under tilted magnetic field. In addition, a strong positive and temperature-dependent magnetoresistance is observed. The transport measurements are analyzed by a model based on two parallel conductive channels.