Electronic structure of superconducting KC₈ and non-superconducting LiC₆ graphite intercalation compounds: Evidence for a graphene-sheet-driven superconducting state

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We have performed photoemission studies of the electronic structure in LiC₆ and KC₈, a non-superconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in KC₈ than in LiC₆, opposite of what might be expected from their chemical composition. We have also measured the strength of the electron-phonon interaction on the graphene-derived Fermi surface to carbon derived phonons in both materials and found that it follows a universal trend where the coupling strength and superconductivity monotonically increase with the filling of graphene π states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.