Quantised lattice vibrations (i.e., phonons) in solids are robust and
unambiguous fingerprints of crystal structures and of their symmetry
properties. In metals and semimetals, strong electron-phonon coupling may lead
to so-called Kohn anomalies in the phonon dispersion, providing an image of the
Fermi surface in a non-electronic observable. Kohn anomalies become prominent
in low-dimensional systems, in particular in graphene, where they appear as
sharp kinks in the in-plane optical phonon branches. However, in spite of
intense research efforts on electron-phonon coupling in graphene and related
van der Waals heterostructures, little is known regarding the links between the
symmetry properties of optical phonons at and near Kohn anomalies and their
sensitivity towards the local environment. Here, using inelastic light
scattering (Raman) spectroscopy, we investigate a set of custom-designed
graphene-based van der Waals heterostructures, wherein dielectric screening is
finely controlled at the atomic layer level. We demonstrate experimentally and
explain theoretically that, depending exclusively on their symmetry properties,
the two main Raman modes of graphene react differently to the surrounding
environment. While the Raman-active near-zone-edge optical phonons in graphene
undergo changes in their frequencies due to the neighboring dielectric
environment, the in-plane, zone-centre optical phonons are symmetry-protected
from the influence of the latter. These results shed new light on the unique
electron-phonon coupling properties in graphene and related systems and provide
invaluable guidelines to characterise dielectric screening in van der Waals
heterostructures and moir\'e superlattices.Comment: 22 pages, main manuscript with 3 figures, appendix with 5 figures,
supplemental material with 6 figure