Superconducting Metals in Porous Glass as Granular Superconductors

The critical fields of superconductors in porous glass are found to have the same temperature dependence as the critical field of a homogeneous dirty alloy. An effective mean free path for electrons in the normal state can be deduced; it is proportional to the pore diameter but is much smaller. This is most easily explained if the superconductor consists of grains separated by tunneling barriers, for this gives a mean free path equal to the grain size multiplied by an average of the transmission coefficient of the barriers. This model can also give a plausible explanation of the large change in critical field produced by chemical treatment of the glass before impregnation with metal. Parmenter's theory of granular superconductors shows that vortices can exist in such materials, and the critical current should be determined by vortex pinning. The measured critical current for indium in porous glass is less than that calculated from a Parmenter formula which assumes that the fields in the sample are uniform.