Phonon-Induced Gaps in Graphene and Graphite Observed by Angle-Resolved Photoemission

Mapping by angle-resolved photoemission spectroscopy of the spectral functions of graphite and graphene layers at low temperatures reveals a heretofore unreported gap of similar to 67 meV at normal emission. This gap persists to room temperature and beyond, and diminishes for increasing emission angles. We show that this gap arises from electronic coupling to out-of-plane vibrational modes at the (K) over bar point in the surface Brillouin zone in accordance with conservation laws and selection rules governed by quantum mechanics. Our study suggests a new approach for characterizing phonons and electron-phonon coupling in solids.