A detailed description is given of the preparation of granular aluminum films (grain sizes less than 40 Å) with stable superconducting transition temperatures $T_c$ up to 3.7°K. We present new experimental data of energy gaps, parallel critical fields, and critical current densities. The results are in good agreement with the predictions of the theories of granular superconductors. However, it is found that none of the mechanisms offered so far for the enhancement of $T_c$ can satisfactorily account for all the properties of the enhanced $T_c$ granular films.

• Received 13 November 1967

DOI:https://doi.org/10.1103/PhysRev.168.444