Understanding electron-phonon interactions in doped graphene: the case of Li-intercalated graphite

The recent explosion of research on doped graphene systems together with the discovery of superconductivity in CaC$_6$ has reignited the interest in graphite intercalation compounds (GICs). While it is generally agreed that the superconductivity observed in GICs is BCS-like, there is still much controversy over which electrons and which phonons are primarily involved in the electron-phonon (e-ph) coupling leading to superconductivity. Moreover, thanks to the close similarity between the electronic structure of GICs and doped graphene, the study of e-ph interactions in GICs provides a unique approach to help elucidate the complex e-ph interactions in doped graphitic systems. We present inelastic x-ray scattering measurements of the high energy ($\sim$ 200 meV) graphitic phonons in LiC$_6$ across the Brillouin zone. The LiC$_6$ phonons are much softer than in pure graphite, as the electron doping destabilizes the C-C bonds. We observe large phonon broadening for all phonons at the graphite Brillouin zone center, suggestive of unusual e-ph interaction phenomena. We discuss our results in the light of the e-ph coupling reported from angle-resolved photoemission spectroscopy and in relation to strong non-adiabatic effects observed using Raman scattering.