Ab initio calculations of H$_{c2}$ for Nb, NbSe2, and MgB2

TL;DR

This study uses ab initio electronic structure calculations to accurately predict the upper critical magnetic field in Nb, NbSe2, and MgB2 superconductors, highlighting the importance of Fermi surface anisotropy.

Contribution

First-principles calculations of $H_{c2}$ for superconductors that match experimental data without adjustable parameters, emphasizing Fermi surface effects.

Findings

Excellent reproduction of $H_{c2}$ temperature and directional dependence for Nb and NbSe2.

Good fit for MgB2 with a $ ext{pi}/ ext{sigma}$ gap ratio of ~0.3.

Fermi surface anisotropy is crucial for accurate $H_{c2}$ modeling.

Abstract

We report on quantitative calculations of the upper critical field $H_{c2}$ for clean type-II superconductors Nb, NbSe$_{2}$, and MgB$_{2}$ using Fermi surfaces from {\em ab initio} electronic structure calculations. The results for Nb and NbSe$_2$ excellently reproduce both temperature and directional dependences of measured $H_{c2}$ curves without any adjustable parameters, including marked upward curvature of NbSe$_{2}$ near $T_{c}$. As for MgB$_2$, a good fit is obtained for a $\pi / \sigma$ gap ratio of $\sim 0.3$, which is close to the value from a first-principles strong-coupling theory [H. J. Choi \textit{et al}. Nature, \textbf{418} 758 (2002)]. Our results indicate essential importance of Fermi surface anisotropy for describing $H_{c2}$.