Heat-capacity measurements on granular aluminum

Abstract

We have measured the heat capacity of a series of films of aluminum in ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$, with normalstate resistivities ${\rho }_N$ from 2 × ${10}^{-5\mathrm{}}$ Ω cm to 4 × ${10}^{-2\mathrm{}}$ Ω 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 ${\rho }_N$ and diminishes as ${\rho }_N$ becomes greater than ${10}^{-3\mathrm{}}$ Ω cm, until it is no longer observable for the highest ${\rho }_N$. We conclude that as ${\rho }_N$ 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.