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arXiv · 2303.13602

Electron-phonon coupling and competing Kekulé orders in twisted bilayer graphene

Yves H. Kwan·Glenn Wagner·Nick Bultinck·Steven H. Simon·Erez Berg·S. A. Parameswaran
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Abstract

Recent scanning tunneling microscopy experiments [K.P. Nuckolls et al., arXiv:2303.00024] have revealed the ubiquity of Kekulé charge-density wave order in magic-angle twisted bilayer graphene. Most samples are moderately strained and show `incommensurate Kekulé spiral' (IKS) order involving a graphene-scale charge density distortion uniaxially modulated on the scale of the moiré superlattice, in accord with theoretical predictions. However, ultra-low strain samples instead show graphene-scale Kekulé charge order that is uniform on the moiré scale. This order, especially prominent near filling factor $ν=-2$, is unanticipated by theory which predicts a time-reversal breaking Kekulé current order at low strain. We show that including the coupling of moiré electrons to graphene-scale optical zone-corner (ZC) phonons stabilizes a uniform Kekulé charge ordered state at $|ν|=2$ with a quantized topological (spin or anomalous Hall) response. Our work clarifies how this phonon-driven selection of electronic order emerges in the strong-coupling regime of moiré graphene.

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Yves H. Kwan, Glenn Wagner, Nick Bultinck, Steven H. Simon, Erez Berg, S. A. Parameswaran. 2023-03-23. Electron-phonon coupling and competing Kekulé orders in twisted bilayer graphene. https://doi.org/10.1103/physrevb.110.085160

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