Theory of superconductivity in graphite intercalation compounds

Abstract

The superconducting transition temperature, T c, of the first stage alkali-graphite intercalation compounds (GICs), especially of C 8K, has been calculated, assuming that the three-dimensional (3D) alkali-like electrons contribute to superconductivity. We suggest that the McMillan formula for T c is applicable to the GIC systems, in contrast to a previous theoretical treatment by Takada. Thus the mass enhancement factor of the 3D electrons due to the electron-phonon interactions is calculated from the first principle. It is confirmed that the interaction with the longitudinal phonon modes in which the carbon and alkali metal atoms vibrate along the c-axis is very strong, while that with the phonon modes in which all the atoms vibrate along the layers is screened by the two-dimensional, graphite-like electrons. The former mechanism is characteristic of GICs and this results in superconductivity in GICs. The latter, i.e., the screening effect due to the 2D electrons, is mainly responsible for the reduction of T c by pressure, recently observed in alkali-amalgam GICs.