Ginzburg‐Landau Equations and the Upper Critical Magnetic Field in Graphite Intercalation Compounds

The Ginzburg‐Landau equations of a dirty two‐band superconductor, where superconductivity is driven by the interband electron‐phonon interaction are derived. On using these Ginzburg‐Landau equations, the obtained formulas for the critical field H c2 describe well the temperature and angular dependences of H c2 in graphite intercalation compounds (GICs). The calculated H   c2 ⟂ ∥ for C 6 K show a linear temperature dependence in a broad temperature range up to the values of the reduced temperature t = T / T c ≈ 0.3 which is a general feature of GICs.