Screening of electron-phonon coupling in graphene on Ir(111)

The phonon dispersion of graphene on Ir(111) has been determined by means of angle-resolved inelastic electron scattering and density functional calculations. Kohn anomalies of the highest optical-phonon branches are observed at the $\Gamma$ and K point of the surface Brillouin zone. At K the Kohn anomaly is weaker than observed from pristine graphene and graphite. This observation is rationalized in terms of a decrease of the electron-phonon coupling due to screening of graphene electron correlations by the metal substrate.