Plasmon-phonon coupling in graphene

Collective excitations of coupled electron-phonon systems are calculated for both monolayer and bilayer graphenes, taking into account the nonperturbative Coulomb coupling between electronic excitations in graphene and the substrate longitudinal-optical phonon modes. We find that the plasmon-phonon coupling in monolayer graphene is strong at all densities but in bilayer graphene the coupling is significant only at high densities satisfying the resonant condition ω_pl ≈ ω_ph. The difference arises from the peculiar screening properties associated with chirality of graphene. Plasmon-phonon coupling explains the measured quasilinear plasmon dispersion in the long-wavelength limit, thus resolving a puzzle in the experimental observations.