  Van der Waals heterostructures, vertical stacks of layered materials, offer
newopportunities for novel quantum phenomena which are absent in their
constituent components. Here we report the emergence of polaron quasiparticles
at the interface of graphene/hexagonal boron nitride (h-BN) heterostructures.
Using nanospot angle-resolved photoemission spectroscopy, we observe
zone-corner replicas of h-BN valence band maxima, with energy spacing
coincident with the highest phonon energy of the heterostructure|an indication
of Frohlich polaron formation due to forward scattering electron-phonon
coupling. Parabolic fitting of the h-BN bands yields an effective mass
enhancement of ~ 2.3, suggesting an intermediate coupling strength. Our
theoretical simulations based on Migdal-Eliashberg theory corroborate the
experimental results, allowing the extraction of microscopic physical
parameters. Moreover,renormalisation of graphene $\pi$ band is observed due to
the hybridisation with the h-BN band. Our work generalises the polaron study
from transition metal oxides to Van derWaals heterostructures with higher
material exibility, highlighting interlayer coupling as an extra degree of
freedom to explore emergent phenomena.
