Heat-capacity measurements on granular aluminum

We have measured the heat capacity of a series of films of aluminum in Al2O3, with normalstate resistivities N from 2 × 10-5 cm to 4 × 10-2 cm. The lattice heat capacity is greater than for the separate bulk constituents, but there is no evidence for a change in the electronic specific heat capacity. The specimens become superconducting with a heat-capacity transition which is BCS-like for low N and diminishes as N becomes greater than 10-3 cm, until it is no longer observable for the highest N. We conclude that as N increases the grains become decoupled. The size of the grains is such that thermodynamic fluctuations prevent the existence of bulk superconducting properties when they are isolated. © 1980 The American Physical Society.