Electronic Structure of Superconducting KC_8 and Nonsuperconducting LiC_6 Graphite Intercalation Compounds: Evidence for a Graphene-Sheet-Driven Superconducting State

摘要

We have performed photoemission studies of the electronic structure in LiC_6 and KC_8, 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 KC_8 than in LiC_6, 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.