The Upper Critical Field of Superconducting Alloys

There existed quantitative descrepancies in the upper critical field Hc2 of type-II superconductors between experimental observations and theory which as sumes the spherical Fermi surface and which is based on the BCS weak coupling model (GLAG theory). Werthamer and McMillan 1 l have investigated the correction due to the strong-coupling effects and concluded that they make up only a negligible portion of the descrepancy between the weak-coupling theory and experimental observations if one considers a normalized field h = -He~ j(dHc2/ dt)t=l· Meanwhile, Reed et al. 2 l have observed that Hc2 has anisotropic nature in several type-II superconductors which have a cubic crystal structure, and that the relative anisotropy of Hc2 decreases as the reduced temperature t increases and approaches zero as t goes to unity. These natures were attributed to the Fermi surface anisotropy produced by the crystal structure and to the non-local correc tion to the GL-equation by Hohenberg and W erthamer. 3 l Moreover, they found that their Hc2 curves are in better agreement with the experimental observations on Nb and V. Farrell et al. 4 l have made precise measurements of Hc2 for Nb crystals and analyzed their result assuming a four-term Kubic harmonic expression ( 4-parameter