  Kohn anomalies in three-dimensional metallic crystals are dips in the phonon
dispersion that are caused by abrupt changes in the screening of the ion-cores
by the surrounding electron-gas. These anomalies are also present at the
high-symmetry points \Gamma and K in the phonon dispersion of two-dimensional
graphene, where the phonon wave-vector connects two points on the Fermi
surface. The linear slope around the kinks in the highest optical branch is
proportional to the electron-phonon coupling. Here, we present a combined
theoretical and experimental study of the influence of the dielectric substrate
on the vibrational properties of graphene. We show that screening by the
dielectric substrate reduces the electron-phonon coupling at the high-symmetry
point K and leads to an up-shift of the Raman 2D-line. This results in the
observation of a Kohn anomaly that can be tuned by screening. The exact
position of the 2D-line can thus be taken also as a signature for changes in
the (electron-phonon limited) conductivity of graphene.
