Interaction-driven band flattening and correlated phases in twisted bilayer graphene

Creators: Choi, Youngjoon; Kim, Hyunjin; Lewandowski, Cyprian; Peng, Yang; Thomson, Alex; Polski, Robert; Zhang, Yiran; Watanabe, Kenji; Taniguchi, Takashi; Alicea, Jason; Nadj-Perge, Stevan

Abstract

Flat electronic bands, characteristic of 'magic-angle' twisted bilayer graphene, host many correlated phenomena. Nevertheless, many properties of these bands and emerging symmetry-broken phases are still poorly understood. Here we use scanning tunnelling spectroscopy to examine the evolution of the twisted bilayer graphene bands and related gapped phases as the twist angle between the two graphene layers changes. We detect filling-dependent flattening of the bands that is appreciable even when the angle is well above the magic angle value and so the material is nominally in a weakly correlated regime. Upon approaching the magic angle, we further show that the most prominent correlated gaps begin to emerge when band flattening is maximized around certain integer fillings of electrons per moiré unit cell. Our observations are consistent with a model that suggests that a significant enhancement of the density of states caused by the band flattening triggers a cascade of symmetry-breaking transitions. Finally, we explore the temperature dependence of the cascade and identify gapped features that develop in a broad range of band fillings where superconductivity is expected. Our results highlight the role of interaction-driven band flattening in defining the electronic properties of twisted bilayer graphene.

Additional Information

© 2021 Nature Publishing Group. Received 03 February 2021; Accepted 16 August 2021; Published 04 November 2021. The authors acknowledge discussions with F. Guinea, F. von Oppen, and G. Refael. Funding: This work has been primarily supported by NSF grants DMR-2005129 and DMR-172336; and Army Research Office under Grant Award W911NF17-1-0323. Part of the STM characterization has been supported by NSF CAREER programme (DMR-1753306). Nanofabrication efforts have been in part supported by DOE-QIS programme (DE-SC0019166). S.N.-P. acknowledges support from the Sloan Foundation. J.A. and S.N.-P. also acknowledge support of the Institute for Quantum Information and Matter, an NSF Physics Frontiers Center with support of the Gordon and Betty Moore Foundation through Grant GBMF1250; C.L. acknowledges support from the Gordon and Betty Moore Foundation's EPiQS Initiative (grant GBMF8682). A.T. and J.A. are grateful for the support of the Walter Burke Institute for Theoretical Physics at Caltech. Y.P. acknowledges support from the startup fund from California State University, Northridge. Y.C. and H.K. acknowledge support from the Kwanjeong Fellowship. Data availability: The data reported in Figs. 1–4 can be found on zenodo: https://zenodo.org/record/5173159. Other data that support the findings of this study are available from the corresponding authors on reasonable request. Code availability: The code that supports the findings of this study is available from the corresponding authors on reasonable request. These authors contributed equally to this work: Youngjoon Choi, Hyunjin Kim. Author Contributions: Y.C. and H.K. fabricated samples with the help of R.P. and Y.Z., and performed STM measurements. Y.C., H.K. and S.N.-P. analysed the data. C.L. and Y.P. implemented TBG models. C.L., Y.P. and A.T. provided theoretical analysis of the model results supervised by J.A. S.N.-P. supervised the project. Y.C., H.K., C.L., Y.P., A.T., J.A. and S.N.-P. wrote the manuscript with input from other authors. The authors declare no competing interests. Peer review information: Nature Physics thanks the anonymous reviewers for their contribution to the peer review of this work.

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