Graphite intercalation compound KC$_8$ revisited: a key to graphene

TL;DR

This study revisits the electronic structure of the graphite intercalation compound KC$_8$, demonstrating that it preserves Dirac Fermion behavior and elucidating the electron-phonon interactions responsible for its superconductivity.

Contribution

It provides a detailed experimental and theoretical analysis showing negligible interlayer coupling in KC$_8$ and identifies the phonon-mediated mechanism for its superconductivity.

Findings

Excellent agreement between experiment and GW calculations of doped graphene

KC$_8$ exhibits negligible interlayer coupling, preserving Dirac Fermion behavior

Superconductivity in KC$_8$ is mediated by electron-phonon coupling to an iTO phonon

Abstract

Electrons in isolated graphene layers are a two-dimensional gas of massless Dirac Fermions. In realistic devices, however, the electronic properties are modified by elastic deformations, interlayer coupling and substrate interaction. Here we unravel the electronic structure of doped graphene, revisiting the stage one graphite intercalation compound KC$_8$ using angle--resolved photoemission spectroscopy and ab--initio calculations. The full experimental dispersion is in excellent agreement to calculations of doped graphene once electron correlations are included on the $GW$ level. This highlights that KC$_8$ has negligible interlayer coupling. Therefore Dirac Fermion behaviour is preserved and we directly determine the full experimental Dirac cone of doped graphene. In addition we prove that superconductivity in KC$_8$ is mediated by electron--phonon coupling to an iTO phonon, yielding a strong kink in the quasiparticle dispersion at 166 meV. These results are key for understanding, both, the unique electronic properties of graphene and superconductivity in KC$_8$.