Experimental evidence of the Coulomb gap in a high-mobility 2D electron system in silicon

In some intermediate temperature range, for low electron densities and zero magnetic field, the resistivity of a two-dimensional electron system in silicon was found to follow the form rho=rho(0) exp [(T-0/T)(1/2)] for at least four orders of magnitude. In addition, in this same region, it was found that in the low T limit, the resistivity follows rho=rho(1) exp [(E(0)/E)(1/2)], where E is the electric field. These dependencies are characteristic of a Coulomb gap in the 2D density of states. We see no evidence of a temperature-dependent or electric-field dependent prefactor. A comparison with the theory shows that there exists a specific set of conditions which are necessary in order to observe this Coulomb gap.