Dielectric screening of the Kohn anomaly of graphene on hexagonal boron nitride

TL;DR

This study demonstrates how dielectric screening from substrates influences the Kohn anomaly and electron-phonon interactions in graphene, affecting its vibrational properties and Raman spectral features.

Contribution

It provides a combined theoretical and experimental analysis of substrate dielectric effects on graphene's phonon dispersion and electron-phonon coupling.

Findings

Screening reduces electron-phonon coupling at K point.

Up-shift of the Raman 2D-line due to dielectric screening.

Kohn anomaly can be tuned by substrate dielectric properties.

Abstract

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.