EXPERIMENTAL-EVIDENCE FOR A COULOMB GAP IN 2 DIMENSIONS

We have studied the resistivity of a two-dimensional electron system i n 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 i nsulating regime. Our results show that at an intermediate temperature range, rho = rho(0) exp [(T-0/T)1/2] for at least four orders of magn itude up to 3 x 10(9) Omega. This behavior is consistent with the exis tence of a Coulomb gap. Near the metal/insulator transition, the prefa ctor was found to be p(0) approximate to h/e(2), and resistivity scale s with temperature. For very low electron densities n(s), the prefacto r diminishes with diminishing n(s). A comparison with the theory shows that a specific set of conditions are necessary to observe the behavi or of resistivity consistent with the existence of the Coulomb gap.