Nearly symmetric field effect in Ultrathin metal films

Ultrathin films of Bi and Pb have been grown incrementally under ultra-high vacuum conditions on SrTiO3 substrates held below 10K. The substrates were provided with metal gates on their rear surfaces. For films with sheet resistance in excess of the quantum resistance for pairs, the conductance was found to increase with gate voltage of either sign. This response, which was temporally stable, was largest in the most resistive films where the conductance changed by the order of 50 percent with a gate bias of 20V. The effect decreased in magnitude with decreasing sheet resistance and increasing temperature. It vanished above a characteristic temperature, and was not present in superconducting films with sheet resistances (measured at 14K) less than the quantum resistance for pairs.