Recent measurements of the anisotropy of the upper critical field BC2 on MgB2 single crystals had revealed a puzzlingly strong temperature dependence. Here, calculations were made of the upper critical field, based upon a detailed modeling of band structure calculations that took into account both the unusual Fermi surface topology and the 2-gap nature of the superconducting order parameter. The results showed that the strong temperature dependence of the BC2 anisotropy could be understood as an interplay of the dominating gap on the σ band, which possessed a small c-axis component of the Fermi velocity, with the induced superconductivity on the π-band possessing a large c-axis component of the Fermi velocity. Analytical formulae were provided for the anisotropy ratio at T = 0 and T = TC and quantitative predictions were made of the distortion of the vortex lattice, based upon the present calculations.

Fermi Surface Topology and the Upper Critical Field in Two-band Superconductors - Application to MgB2. T.Dahm, N.Schopohl: Physical Review Letters, 2003, 91[1], 017001 (3pp)