Anisotropy of the Upper Critical Field in Superconductors with Anisotropic Gaps: Anisotropy Parameters of MgB₂

Abstract

The upper critical field H_c2 is evaluated for weakly-coupled two-band anisotropic superconductors. By modeling the actual bands and the gap distribution of MgB₂ by two Fermi surface spheroids with average parameters of the real material, we show that H_c2,ab⁄H_c2,c increases with decreasing temperature in agreement with available data.