Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces

Xiaoyu Wang; Joe Finney; Aaron L. Sharpe; Linsey K. Rodenbach; Connie L. Hsueh; Kenji Watanabe; Takashi Taniguchi; M. A. Kastner; Oskar Vafek; David Goldhaber-Gordon

doi:10.1073/pnas.2307151120

Article Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces

Xiaoyu Wang ; Joe Finney ; Aaron L. Sharpe ; Linsey K. Rodenbach ; Connie L. Hsueh ; Kenji Watanabe (National Institute for Materials Science) ; Takashi Taniguchi (National Institute for Materials Science) ; M. A. Kastner ; Oskar Vafek ; David Goldhaber-Gordon

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Xiaoyu Wang, Joe Finney, Aaron L. Sharpe, Linsey K. Rodenbach, Connie L. Hsueh, Kenji Watanabe, Takashi Taniguchi, M. A. Kastner, Oskar Vafek, David Goldhaber-Gordon. Unusual magnetotransport in twisted bilayer graphene from strain-induced open Fermi surfaces. Proceedings of the National Academy of Sciences. 2023, 120 (34), e2307151120. https://doi.org/10.1073/pnas.2307151120

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Anisotropic hopping in a toy Hofstadter model was recently invoked to explain a rich and surprising Landau spectrum measured in twisted bilayer graphene away from the magic angle. Suspecting that such anisotropy could arise from unintended uniaxial strain, we extend the Bistritzer-MacDonald model to include uniaxial heterostrain. We find that such strain strongly influences band structure, shifting the three otherwise-degenerate van Hove points to different energies. Coupled to a Boltzmann magnetotransport calculation, this reproduces previously-unexplained non-saturating B2 magnetoresistance, and predicts subtler features that had not been noticed in the experimental data. In contrast to these distinctive signatures in longitudinal resistivity, the Hall coefficient is barely influenced by strain, to the extent that it still shows a single sign change on each side of the charge neutrality point. The theory also predicts a marked rotation of the electrical transport principal axes with changing of filling even for fixed strain and for rigid bands. Such filling-dependent rotations have led researchers to reject strain as a cause, instead invoking electronic nematic order.

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