Superconducting graphene sheets in CaC6 enabled by phonon-mediated interband interactions

There is a great deal of fundamental and practical interest in the possibility of inducing superconductivity in a monolayer of graphene. But while bulk graphite can be made to superconduct when certain metal atoms are intercalated between its graphene sheets, the same has not been achieved in a single layer. Moreover, there is a considerable debate about the precise mechanism of superconductivity in intercalated graphite. Here we report angleresolved photoelectron spectroscopy measurements of the superconducting graphite intercalation compound CaC6 that distinctly resolve both its intercalant-derived interlayer band and its graphene-derived pi* band. Our results indicate the opening of a superconducting gap in the pi* band and reveal a substantial contribution to the total electron-phonon-coupling strength from the pi*-interlayer interband interaction. Combined with theoretical predictions, these results provide a complete account for the superconducting mechanism in graphite intercalation compounds and lend support to the idea of realizing superconducting graphene by creating an adatom superlattice.