Experimental evidence for a Coulomb gap in two dimensions

We have studied the resistivity of a two-dimensional electron system in silicon in the temperature range 200 mK <T<7.5 K at zero magnetic field at low electron densities, when the electron system is in the insulating regime. Our results show that at an intermediate temperature range, ρ=${\mathrm{\rho }}_0$exp[(${\mathit{T}}_0$/T${)}^{1/2}$] for at least four orders of magnitude up to 3×${10}^9$ Ω. This behavior is consistent with the existence of a Coulomb gap. Near the metal/insulator transition, the prefactor was found to be ${\mathrm{\rho }}_0$≊h/${\mathit{e}}^2$, and resistivity scales with temperature. For very low electron densities ${\mathit{n}}_{\mathit{s}}$, the prefactor diminishes with diminishing ${\mathit{n}}_{\mathit{s}}$. A comparison with the theory shows that a specific set of conditions is necessary to observe the behavior of resistivity consistent with the existence of the Coulolmb gap.