Clarifying the apparent flattening of the graphene band near the van Hove singularity

Graphene band renormalization near the van Hove singularity (VHS) has been investigated by angle-resolved photoemission spectroscopy (ARPES) on Li-doped quasifreestanding graphene on a cobalt (0001) surface. The absence of graphene band hybridization with the substrate, the doping contribution well represented by a rigid energy shift, and the excellent electron-electron interaction screening ensured by the metallic substrate offer a privileged point of view for such an investigation. A clear ARPES signal is detected along the KMK direction of the graphene Brillouin zone, giving rise to an apparent flattened band. By simulating the graphene spectral function from the density functional theory calculated bands, we demonstrate that the photoemission signal around the M point originates from the "tail"of the spectral function of the unoccupied band above the Fermi level. Such an interpretation puts forward the absence of any additional strong correlation effects near the VHS, reconciling the mean-field description of the graphene band structure even in a highly doped scenario.