Inhomogeneous superconducting transitions in granular Al

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 Tcρ, where R(T)0. Microwave loss has been identified which appears to be specific to granular materials and vanishes only well below Tcρ. This is shown to be due to normal conduction between grains, which upon cooling becomes progressively more Josephson-like. © 1980 The American Physical Society.