High magnetic fields and low-temperature peculiarities of the properties of highly disordered two-dimensional systems

Informacje na temat projektu

Identyfikator umowy o grant: INTAS-93-1403

Projekt został zamknięty

Data rozpoczęcia 1 Lipca 1994

Data zakończenia 30 Czerwca 1995

Finansowanie w ramach

• International Association for the promotion of cooperation with scientists from the independent states of the former Soviet Union (INTAS), 1993-

Koszt całkowity € 0,00

Wkład UE € 34 000,00

Koordynowany przez

Centre National de la Recherche Scientifique
France

Exploitable results

High magnetic fields and low-temperature peculiarities of the properties of highly disordered two-dimensional systems

The low-temperature properties of semiconductors are determined by quantum coherent effects. However, recent studies of transport properties in the extreme quantum limit (EQL) and in highly disordered systems revealed new additional peculiarities which are not yet explained. The main aim of this project was to study the influence of common action of high magnetic field, especially extreme quantum limit, and strong disorder on quantum transport effects in 2D systems. Magnetotransport characteristics of dilute 2D hole gases in Si/SiGe heterostructures at low temperatures are shown to have anomalous behaviour. With increasing magnetic field, the normal sequence of QHE states is interrupted by the appearance of the insulating state in the region of filling factor two, which in turn is replaced by the i=1 QHE state. A new model which describes quantum transport in terms of percolation phenomena, predicting a nonmonotonic dependence of Hall conductivity on magnetic field, is shown to be capable of explaining the data. Surface acoustic wave (SAW) studies of QHE in disordered GaAs/GaAlAs structures showed that for small semi-integer filling factors there exists a region near the Landau zone centre, where the σxxac = σxxdc- electrons are delocalised. However, for the integer filling factors this does not hold and σxxac always exceeds σxxdc. This leads one to the conclusion that in quantum Hall regime electrons are localized, and lack of the σxxac dependence on frequency shows that electrons are localized in long range fluctuation potential. Non-linear behaviour of SAW attenuation in the QHE regime was studied. For delocalised electrons the electron gas temperature Te was determined and it was found that the energy relaxation takes place via the piezoacoustic phonons. The few phenomena in which conductance of macroscopic structure is determined by one or few regions of mesoscopic size were observed, in particular: In compensated semiconductors near magnetic field induced metal-insulator transition we observed peculiarities of temperature dependece of conductivity which was found to be due to superlocalisation; In Si-MOS structures with quazi-2D hopping conductivity mesoscopic fluctuations of voltage between hall probes were detected and studied in spite of macroscopic structure size; In short-channel gated structures with the high concentration of built-in charge there was observed peculiarities of conductance G dependence on gate voltage Vg related to transition to the quasi-1D conductivity. That leads to quantum behaviour of G, while in structures of high sizes the net of quasi-1D channels results in threshold for Hall effect at G~e2/h. The reversible conversion of high frequency electric fields into longitudinal acoustic waves in modulation-doped pseudomorphic Si/SiGe heterostructures was demonstrated being the evidence for the existence of a piezoelectric coupling within such structure. New type of ordering in InxGa1-xP (with x=0.5) grown by MOS-hybride epitaxy on (110) GaAs has been observed. In the ordered phase the In and Ga atoms occupy cation sites not randomly but rather in pairwise alternating (110) planes.