The influence of band structure on the electromagnetic properties of superconducting Nb and Nb3Sn

This study examines two parameters describing the electromagnetic properties of superconductors: the upper critical field Hc2 and the low-field penetration depth lambda . The current state of the theory for Hc2 is summarised and some new results for lambda are derived to focus on the dominant role of the Fermi surface topology and resultant anisotropies. These properties are evaluated for Nb and Nb3Sn as specific cases. Global parameters are found to be in good agreement with experimental data but other properties exhibit some scatter. The anisotropic deviations of the electromagnetic response are given to about 10%. An estimate is obtained for the dependence of the penetration depth on the direction of the shielding current.