Quantum Phase Slips and Superconductivity in Granular Films

Abstract
An explanation is proposed for the recent observation that in ultrathin Sn films there is apparently a universal normal-state sheet resistance above which superconductivity cannot be established. It is demonstrated that when the sheet resistance is larger than the quantum of resistance, RQ=h4e2, the order parameter suffers phase slips due to quantum tunneling driving the film normal at low temperature. This value of the resistance agrees quantitatively with experiment.