Andrej M. Kreshchuk

Quantum transport effects in a two-dimensional electron gas as a tool for the investigation of heterointerfaces

We have studied the influence of interface defects on low temperature galvanomagnetic effects in a two-dimensional electron gas (2DEG). Experiments have shown a non-ideal character of the InP/In 0.53 Ga 0.47 As interface in modulation-doped structures with 2DEG grown by liquid phase epitaxy. That is why such structures have been chosen for investigation. The following galvanomagnetic effects were investigated at liquid helium temperatures: the Hall mobility; the oscillatory magnetoresistance in a magnetic field perpendicular to the heterointerface; the negative magnetoresistance in a weak magnetic field parallel to 2DEG. The quantitative analysis of these effects allowed one to determine the interface parameters: the amplitude δ=7-10 A and the lateral size Λ=50-70 A of small-scale interface fluctuation, the interface charge density N z =(8±2) ×10 19 cm -2 and the amplitude of large-scale 2DEG density fluctuations δn s /n s 5%

Weak-field magnetoresistance of two-dimensional electrons in In0.53Ga0.47As/InP heterostructures in the persistent photoconductivity regime

The weak-field magnetoresistance of the two-dimensional electron gas (2DEG) in a modulation-doped In0.53Ga0.47As/InP heterostructure is studied as the state of the system is converted to a state with persistent photoconductivity by illuminating the sample with interband light. The concentration dependences of the parameters that characterize the phase (Hϕ) and spin (Hs coherence are investigated, both in the low-concentration regime where only the first quantum-well subband is occupied by carriers, and in the regime where the second subband is occupied. A qualitative description of all the features observed in experiment is obtained by taking into account the redistribution of charge in the persistent photoconductivity state and the importance of processes that take place in the second quantum-well subband even when its occupation is small.

Spin relaxation and weak localization of two-dimensional electrons in asymmetric quantum Wells

The anomalous alternating-sign magnetoresistance in a two-dimensional electron gas on an In0.53Ga0.47As/InP heterostructure was investigated experimentally at liquid-helium temperatures in a wide range of electron densities, including the case of two filled quantum-well subbands. The data obtained are analyzed in terms of a theory that takes into account terms in the spin splitting of the electron spectrum which are cubic and linear in the wave vector. The linear term is related to the asymmetry of the quantum well, i.e., the presence of an electric field at the heterojunction. It is shown that the new theoretical model describes the experiment better.

Electron localization in sound absorption oscillations in the quantum Hall effect regime

The absorption coefficient for surface acoustic waves in a piezoelectric insulator in contact with a GaAs/Al0.25Ga0.75As heterostructure (with two-dimensional electron mobility μ=1.3×105 cm2/(V·s) at T=4.2 K) via a small gap has been investigated experimentally as a function of the frequency of the wave, the width of the vacuum gap, the magnetic field, and the temperature. The magnetic field and frequency dependences of the high-frequency conductivity (in the region 30–210 MHz) are calculated and analyzed. The experimental results can be explained if it assumed that there exists a fluctuation potential in which current carrier localization occurs. The absorption of the surface acoustic waves in an interaction with two-dimensional electrons localized in the energy “tails” of Landau levels is discussed.

The Nonlinear Effects in 2DEG Conductivity Investigation by an Acoustic Method

The parameters of two-dimensional electron gas (2DEG) in a GaAs/AlGaAs het-erostructure were determined by an acoustical (contactless) method in the delocalized electrons region (B ≤2.5T). Nonlinear effects in Surface Acoustic Wave (SAW) absorption by 2DEG are determined by the electron heating in the electric field of SAW, which may be described in terms of electron temperature T e. The energy relaxation time τ ǫ is determined by the scattering at piezoelectric potential of acoustic phonons with strong screening. At different SAW frequencies the heating depends on the relationship between ωτ ǫ and 1 and is determined either by the instantaneously changing wave field (ωτ ǫ < 1), or by the average wave power (ωτ ǫ > 1).