Anisotropy of the upper critical field in MgB2: the two-gap Ginzburg-Landau theory

TL;DR

This paper investigates the anisotropic upper critical field in MgB2 using a two-gap Ginzburg-Landau theory, revealing how temperature affects gap distributions and critical field behavior.

Contribution

It introduces a variational solution to the two-gap Ginzburg-Landau equations that captures the temperature-dependent anisotropy of Hc2 in MgB2.

Findings

Different spatial gap distributions in two bands.

Temperature-dependent ratio of the two gaps influences Hc2 behavior.

Hexagonal in-plane modulations of Hc2 can change sign with temperature.

Abstract

The upper critical field in MgB2 is investigated in the framework of the two-gap Ginzburg-Landau theory. A variational solution of linearized Ginzburg-Landau equations agrees well with the Landau level expansion and demonstrates that spatial distributions of the gap functions are different in the two bands and change with temperature. The temperature variation of the ratio of two gaps is responsible for the upward temperature dependence of in-plane Hc2 as well as for the deviation of its out-of-plane behavior from the standard angular dependence. The hexagonal in-plane modulations of Hc2 can change sign with decreasing temperature.