posted on 2019-11-14, 18:11authored byPeter Rickhaus, Giulia Zheng, Jose L. Lado, Yongjin Lee, Annika Kurzmann, Marius Eich, Riccardo Pisoni, Chuyao Tong, Rebekka Garreis, Carolin Gold, Michele Masseroni, Takashi Taniguchi, Kenji Wantanabe, Thomas Ihn, Klaus Ensslin
Crystal fields occur due to a potential difference between
chemically
different atomic species. In van der Waals heterostructures such fields
are naturally present perpendicular to the planes. It has been realized
recently that twisted graphene multilayers provide powerful playgrounds
to engineer electronic properties by the number of layers, the twist
angle, applied electric biases, electronic interactions, and elastic
relaxations, but crystal fields have not received the attention they
deserve. Here, we show that the band structure of large-angle twisted
double bilayer graphene is strongly modified by crystal fields. In
particular, we experimentally demonstrate that twisted double bilayer
graphene, encapsulated between hBN layers, exhibits an intrinsic band
gap. By the application of an external field, the gaps in the individual
bilayers can be closed, allowing to determine the crystal fields.
We find that crystal fields point from the outer to the inner layers
with strengths in the bottom/top bilayer Eb = 0.13 V/nm ≈ −Et = 0.12 V/nm. We show both by means of first-principles
calculations and low energy models that crystal fields open a band
gap in the ground state. Our results put forward a physical scenario
in which a crystal field effect in carbon substantially impacts the
low energy properties of twisted double bilayer graphene, suggesting
that such contributions must be taken into account in other regimes
to faithfully predict the electronic properties of twisted graphene
multilayers.