Crossover phenomenon for two-dimensional hopping conductivity and density-of-states near the Fermi level

The variable range hopping (VRH) resistivity in a gated delta-doped GaAs/Al GaAs heterostructure was measured at temperatures down to 280 mK. At low te mperatures, the logarithm of the resistivity follows T-1/2 behaviour, which corresponds to the existence of a soft (linear) Coulomb gap in the density of states (DOS) at the Fermi level (FL). As the temperature is increased, there is a crossover to T-1/3 behaviour, corresponding to a constant DOS ou tside the Coulomb gap. A quantitative analysis of the resistivity data allo ws us to determine the width of the Coulomb gap and the DOS around the FL. (C) 1999 Published by Elsevier Science Ltd. All rights reserved.