The variable range hopping (VRH) resistivity in a gated δ-doped GaAs/AlGaAs heterostructure was measured at temperatures down to 280 mK. At low temperatures, 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 outside the Coulomb gap. A quantitative analysis of the resistivity data allows us to determine the width of the Coulomb gap and the DOS around the FL.
Bibliographical noteFunding Information:
This work was supported by the Engineering and Physical Science Research Council (UK). The authors thank A. Khaetskii, M. Kaveh and E. Kogan for fruitful discussions. SIK acknowledges support from the Commonwealth Scholarship Commission in the UK.