Anomalous Field Effect in Ultrathin Films of Metals near the Superconductor-Insulator Transition

A field effect conductance modulation experiment has been performed on a series of nominally homogeneous ultrathin films of metals. The thicknesses of the films were varied over a range such that their properties traversed the insulator-to-superconductor transition. At low gate voltages VG the conductance G(VG) increased with either polarity for films on the insulating side of the transition and decreased at temperatures in the transition region for films which were just thick enough to be superconducting. A qualitative interpretation of these results suggests the consideration of Cooper pairing even for insulating films.