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, $R\left(T\right)\mathrm{}$. The change in resonant frequency is proportional to the number of superconducting electron pairs which is finite above the temperature ${T_c}^{\rho }$, where $R\left(T\right)\mathrm{}\simeq 0$. Microwave loss has been identified which appears to be specific to granular materials and vanishes only well below ${T_c}^{\rho }$. This is shown to be due to normal conduction between grains, which upon cooling becomes progressively more Josephson-like.