Resonance Raman Spectrum of Doped Epitaxial Graphene at the Lifshitz Transition

We employ ultra-high vacuum (UHV) Raman spectroscopy in tandem with angle-resolved photoemission (ARPES) to investigate the doping-dependent Raman spectrum of epitaxial graphene on Ir(111). The evolution of Raman spectra from pristine to heavily Cs doped graphene up to a carrier concentration of 4.4 × 10¹⁴ cm^–2 is investigated. At this doping, graphene is at the onset of the Lifshitz transition and renormalization effects reduce the electronic bandwidth. The optical transition at the saddle point in the Brillouin zone then becomes experimentally accessible by ultraviolet (UV) light excitation, which achieves resonance Raman conditions in close vicinity to the van Hove singularity in the joint density of states. The position of the Raman G band of fully doped graphene/Ir(111) shifts down by ∼60 cm^–1. The G band asymmetry of Cs doped epitaxial graphene assumes an unusual strong Fano asymmetry opposite to that of the G band of doped graphene on insulators. Our calculations can fully explain these observations by substrate dependent quantum interference effects in the scattering pathways for vibrational and electronic Raman scattering.