Fermi surface topology and the upper critical field in two-band superconductors - application to MgB2

TL;DR

This paper models the upper critical field in MgB2 superconductors considering its unique Fermi surface topology and two-gap structure, explaining the temperature-dependent anisotropy observed experimentally.

Contribution

It introduces a detailed calculation method incorporating bandstructure and two-gap effects to explain Bc2 anisotropy in MgB2.

Findings

The temperature dependence of Bc2 anisotropy is explained by the interplay of sigma- and pi-band gaps.

Analytic formulas for anisotropy ratio at T=0 and T=Tc are provided.

Predictions are made for vortex lattice distortion based on the model.

Abstract

Recent measurements of the anisotropy of the upper critical field Bc2 on MgB2 single crystals have shown a puzzling strong temperature dependence. Here, we present a calculation of the upper critical field based on a detailed modeling of bandstructure calculations that takes into account both the unusual Fermi surface topology and the two gap nature of the superconducting order parameter. Our results show that the strong temperature dependence of the Bc2 anisotropy can be understood as an interplay of the dominating gap on the sigma-band, which possesses a small c-axis component of the Fermi velocity, with the induced superconductivity on the pi-band possessing a large c-axis component of the Fermi velocity. We provide analytic formulas for the anisotropy ratio at T=0 and T=Tc and quantitatively predict the distortion of the vortex lattice based on our calculations.