Electronic structure of heavily doped graphene: The role of foreign atom states

Using density functional theory calculations we investigate the electronic structure of graphene doped by deposition of foreign atoms. We demonstrate that, as the charge transfer to the graphene layer increases, the band structure of the pristine graphene sheet is substantially affected. This is particularly relevant when Ca atoms are deposed on graphene at CaC6 stoichiometry. Similarly to what happens in superconducting graphite intercalated compounds, a Ca band occurs at the Fermi level. Its hybridization with the C states generates a strong nonlinearity in one of the pi(*) bands below the Fermi level, at energies comparable to the graphene E-2g phonon frequency. This strong nonlinearity, and not many-body effects as previously proposed, explains the large and anisotropic values of the apparent electron-phonon coupling measured in angular resolved photoemission.