Dielectric Screening Of The Kohn Anomaly Of Graphene On Hexagonal Boron Nitride

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 upshift 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.