Pair Conductivity aboveTcin Aluminum Films

Pair conductivity, as reflected in the temperature dependence of the electrical resistance above the superconducting transition temperature, has been studied in aluminum films in the "two-dimensional" regime, viz., $d\lesssim \xi \mathrm{}\left(T\right)\mathrm{}$, where $d$ is the film thickness and $\xi \mathrm{}\left(T\right)\mathrm{}$ is the superconducting coherence length. Normalstate resistivities were varied over five orders of magnitude, the highest-resistance films having highly disordered granular structures and the lowest-resistance films having mean free paths as large as 400 Å. The predicted temperature dependence of the pair conductivity was found to hold irrespective of sample parameters. However, the prediction that the excess conductivity should be independent of mean free path was found to not hold: Low-resistivity films exhibited an excess conductivity an order of magnitude larger than that predicted by Aslamazov and Larkin.