Theory of Upper Critical Field of Superconducting Alloys

Abstract

The investigation is carried out of the upper critical field H c 2 of a superconducting alloy in the case where an appreciable variation of the transition temperature T c is observed on alloying. It is assumed that the density of states is not a smooth function of energy at and near the Fermi level and that the BCS coupling parameter is anisotropic. It turns out that the shift of Fermi level on alloying does not modify the expression for H c 2 usually employed if one interprets T c and Fermi velocity in the expression as those of the alloy. It is also discussed that a recent investigation by Teichler on the effect of anisotropic coupling ignores an important term. For the case of Nb–Ta system, it becomes clear that the shift of Fermi level is small enough so that Fermi velocity remains constant even though there exists a large variation in T c with alloying. Qualitative discussions are given on the effect of anisotropic coupling, and it turns out that more detailed experimental investigations are needed t...