ANOMALIES IN THE SPECTRA OF QUANTUM OSCILLATIONS FOR DILUTE 2-DIMENSIONAL ELECTRONIC SYSTEMS

Magnetotransport characteristics of a two-dimensional hole gas located at a Si/SiGe heterojunction are measured for two carrier concentratio ns. A notable discrepancy is observed between values of the magnetic l evel filling factor determined from the periodicity of the Shubnikov-d e Haas oscillations and the values of the Hall conductivity expected a t these filling factors. In particular, the sample of lower concentrat ion exhibits a re-entrant transition between the quantum Hall effect s tate and insulating state. It is shown that all the observed effects c an be described by a model [S. I. Dorozhkin, JETP Lett. 60, 595 (1994) ] which takes into account the long-range potential modulation in the sample, with amplitude comparable to the Fermi energy. (C) 1995 Americ an Institute of Physics.