Superconductivity in graphite intercalation compounds

Alkali-metal graphite intercalation compounds undergo a normal-to-superconducting transition at a critical temperature ${\mathit{T}}_{\mathit{c}}$ that increases with increasing alkali-metal concentration. Furthermore, the temperature dependence of the critical magnetic field ${\mathit{H}}_{\mathit{c}2}$ is linear down to very low temperatures. A two-band model, which considers coupling between a three-dimensional band and an almost two-dimensional electronic energy band, and which was successfully applied to explain superconductivity in ${\mathrm{C}}_8$K, is used to interpret recently reported data. In particular, we show that the increase in ${\mathit{T}}_{\mathit{c}}$ as the alkali-metal concentration increases, the linear temperature dependence of ${\mathit{H}}_{\mathit{c}2}$, and the anisotropy in its value, are consistent with the two-band model.