Corrugation-induced electronic effects in twisted bilayer graphene

Relative rotation between the layers of bilayer graphene creates a moiré superlattice with a periodicity that is inversely related to the interlayer twist angle. The interlayer twist angle significantly alters the electronic band structure of the twisted bilayer graphene (tBLG), forming extremely flat bands near the Fermi level at a “magic-angle” of 1.08°. However, the electronic properties in tBLG are extremely sensitive to small structural deformation. Flat bands are observed in rigid tBLG without any lattice relaxation, but this fails to explain the appearance of pseudogap states and spectral weight transfer typically observed in magic-angle tBLG experiments. Here, we show that a large amplitude corrugation, or out-of-plane deformation mode, is thermodynamically stable in low twist angle tBLG due to the competition between in-plane elastic stiffness and out-of-plane bending stiffness. The large corrugation in tBLG alters the band structure, leading to partially filled states and broken symmetry in a narrow range of twist angles near the magic-angle. Our analysis also demonstrates spectral weight transfer in the charge distribution, accompanied by pseudogap states in the magic-angle tBLG with large amplitude corrugation. The large corrugation is consistent with the observed vibrational spectra of tBLG as a function of twist angle. Our observation of large corrugation provides an exciting platform to explore deformation-induced correlated physics in tBLG.