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Crystal fields occur due to a potential difference between chemicallydifferent atomic species. In Van-der-Waals heterostructures such fields arenaturally present perpendicular to the planes. It has been realized recentlythat twisted graphene multilayers provide powerful playgrounds to engineerelectronic properties by the number of layers, the twist angle, appliedelectric biases, electronic interactions and elastic relaxations, but crystalfields have not received the attention they deserve. Here we show that thebandstructure of large-angle twisted double bilayer graphene is stronglymodified by crystal fields. In particular, we experimentally demonstrate thattwisted double bilayer graphene, encapsulated between hBN layers, exhibits anintrinsic bandgap. By the application of an external field, the gaps in theindividual bilayers can be closed, allowing to determine the crystal fields. Wefind that crystal fields point from the outer to the inner layers withstrengths in the bottom (top) bilayer of -0.13 V/nm (0.12 V/nm). We show bothby means of first principles calculations and low energy models that crystalfields open a band gap in the groundstate. Our results put forward a physicalscenario in which a crystal field effect in carbon substantially impacts thelow energy properties of twisted double bilayer graphene, suggesting that suchcontributions must be taken into account in other regimes to faithfully predictthe electronic properties of twisted graphene multilayers.

Gap Opening in Twisted Double Bilayer Graphene by Crystal fields