NEARLY SYMMETRICAL 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 subst rates were provided with metal gates on their rear surfaces. For films with sheet resistance in excess of the quantum resistance for Fairs, 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 perce nt with a gate bias of 20V. The effect decreased in magnitude with dec reasing 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 r esistance for pairs.