Tunneling into granular Pb films in the superconducting and insulating regimes

Abstract

Tunneling measurements have been performed on both sides of the insulator-superconductor transition in quench-condensed granular Pb films. The results indicate that the Pb film consists of fully superconducting grains with essentially bulk values for the superconducting energy gap \ensuremath{\Delta} and transition temperature ${\mathit{T}}_{\mathit{c}}$, even when the film exhibits insulating behavior in transport measurements. The density of states inferred from measurements in perpendicular magnetic field are consistent with magnetic spin-flip broadening.