Ab Initio Calculations of Hc2 for Nb, NbSe2, and MgB2

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  • Arai Masao
    Computational Materials Science Center, National Institute for Materials Science
  • Kita Takafumi
    Division of Physics, Hokkaido University

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  • <i>Ab Initio</i> Calculations of <i>H</i><sub>c2</sub> for Nb, NbSe<sub>2</sub>, and MgB<sub>2</sub>

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We report on the quantitative calculations of the upper critical field Hc2 for clean type-II superconductors Nb, NbSe2, and MgB2 within the weak-coupling theory. The Fermi surface anisotropy is fully taken into account by the energy band data from 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 a 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 that from a first-principles strong-coupling theory [H. J. Choi et al: Nature 418 (2002) 758]. Our results indicate the importance of Fermi surface anisotropy in describing Hc2.

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