Excitonic Effects in Phonons: Reshaping the Graphene Kohn Anomalies and Lifetimes

We develop an ab initio framework that captures the impact of electron-electron and electron-hole interactions on phonon properties. This enables the inclusion of excitonic effects in the optical phonon dispersions and lifetimes of graphene, both near the center (Gamma) and at the border (K) of the Brillouin zone, at phonon-momenta relevant for Raman scattering and for the onset of the intrinsic electrical resistivity. Near K, we find a phonon redshift of similar to 150 cm-1 and a 10 x enhancement of the group velocity, together with a 5 x increase in linewidths due to a 26 x increase of the electron-phonon matrix elements. These effects persist for doping 2EF