We report on quantitative calculations of the upper critical field Hc2 for clean type-II superconductors Nb, NbSe2, and MgB2 using Fermi surfaces from {\em ab initio} electronic structure calculations. The results for Nb and NbSe2 excellently reproduce both temperature and directional dependences of measured Hc2 curves without any adjustable parameters, including marked upward curvature of NbSe2 near Tc. As for MgB2, a good fit is obtained for a π/σ gap ratio of ∼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 Hc2.
@article{arxiv.cond-mat/0404628,
title = {Ab initio calculations of H$_{c2}$ for Nb, NbSe2, and MgB2},
author = {Masao Arai and Takafumi Kita},
journal= {arXiv preprint arXiv:cond-mat/0404628},
year = {2009}
}