Inhomogeneous Superconducting Transitions in Granular Al

Abstract

Granular A1 films have been measured in a microwave cavity at 9.4 GHz together with their dc resistance, $R(T)$. The change in resonant frequency is proportional to the number of superconducting electron pairs which is finite above the temperature ${{T}_{c}}^{\ensuremath{\rho}}$, where $R(T)\ensuremath{\simeq}0$. Microwave loss has been identified which appears to be specific to granular materials and vanishes only well below ${{T}_{c}}^{\ensuremath{\rho}}$. This is shown to be due to normal conduction between grains, which upon cooling becomes progressively more Josephson-like.