Electronic Structure of SuperconductingKC8and NonsuperconductingLiC6Graphite Intercalation Compounds: Evidence for a Graphene-Sheet-Driven Superconducting State

Abstract

We have performed photoemission studies of the electronic structure in ${\mathrm{LiC}}_6$ and ${\mathrm{KC}}_8$, a nonsuperconducting and a superconducting graphite intercalation compound, respectively. We have found that the charge transfer from the intercalant layers to graphene layers is larger in ${\mathrm{KC}}_8$ than in ${\mathrm{LiC}}_6$, 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 ${\pi }^{*}$ states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.