Quantum versus semiclassical analysis of the conductivity of two-dimensional electrons in a magnetic field

Abstract

We present experimental evidence for the levitation of extended states of a two-dimensional electron system in a magnetic field, and establish the presence of alternating regions of localized and extended states in the density of states to Landau-level filling factors (ν) as high as 80. Monitoring the Hall voltage of a modulation-doped (Formula presented) heterojunction at fixed magnetic field during continuous illumination from light-emitting diodes reveals a steplike structure with plateaus at even integer ν between 22 and 80 at a temperature of 0.3 K. We derive a general expression for the conductivity due to rectangular bands of extended states, and show that the observed temperature dependence of the Shubnikov-de Haas oscillations is consistent with this picture. An analysis of the oscillations using this expression reveals the predicted levitation of the extended states. Show more