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 T7.5 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}=ρ₀exp[(T₀/T)1/2] for at least four orders of magnitude up to 3×{}10⁹ {Ω}. This behavior is consistent with the existence of a Coulomb gap. Near the metal/insulator transition, the prefactor was found to be ρ₀{}h/e², and resistivity scales with temperature. For very low electron densities nₛ, the prefactor diminishes with diminishing nₛ. 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.