Tunneling in and the microstructure of granular superconductors

The microstructure of granular superconductors are discussed and the difference between granular and random composites is noted. The model where a granular (and a random) superconductor, close to the percolation threshold, is considered to be made up of large granular dusters, linked by tunnelling junctions or weak links, is introduced. This model is illustrated using recent experimental measurements on nanostructured granular Al-Ge. The measurements dearly show that the coupling junctions between clusters can, depending on the volume fraction of superconductor, be of either a Josephson or quasi-particle nature, or a combination of both. The differences between a granular and a dirty superconductor and the upper critical field of granular superconductors is then discussed and it is shown that the diameter of the grains can be estimated from the upper critical field. Random Al-Ge, which has a rather different microstructure, is also discussed.