Electronic structure of superconducting KC$_8$ and non-superconducting LiC$_6$ graphite intercalation compounds: Evidence for a graphene-sheet-driven superconducting state

TL;DR

This study compares the electronic structures of superconducting KC$_8$ and non-superconducting LiC$_6$ graphite intercalation compounds, revealing that graphene-derived electrons and phonons are key to superconductivity, with electron-phonon coupling correlating with graphene state filling.

Contribution

It provides new insights into the role of graphene-derived electrons and phonons in superconductivity of graphite intercalation compounds, highlighting a universal trend in electron-phonon coupling.

Findings

Charge transfer is larger in KC$_8$ than in LiC$_6$.

Electron-phonon coupling strength increases with graphene $ ext{π}^*$ state filling.

Superconductivity correlates with increased graphene-derived electron-phonon interactions.

Abstract

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 $\pi^{\ast}$ states. This correlation suggests that both graphene-derived electrons and graphene-derived phonons are crucial for superconductivity in graphite intercalation compounds.