Coupling and isolation: Critical field and transition temperature of superconducting granular aluminum

Abstract

Measurements have been made of the critical magnetic field of superconducting granular aluminum ($d\sim 30$ Å) down to temperatures well below $T_c$ ($t^2<0.1\mathrm{}$). The results show that it is possible to have the coupling between grains weak enough and the temperature low enough so that the critical field is that of the isolated grain while at the same time the large fluctuations expected in isolated grains of this size are suppressed. The results for $T_c$, together with those from other sources, indicate that the maximum $T_c$ is a function of grain size only.