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

Nature Physics volume 17, pages 1375–1381 (2021)Cite this article

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Abstract

Flat electronic bands, characteristic of ‘magic-angle’ twisted bilayer graphene, host many correlated phenomena1,2,3,4,5,6,7,8,9. 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.

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Fig. 1: Filling-dependent band structure deformation of TBG at twist angle θ = 1.32°.
Fig. 2: Evolution of LLs with twist angle and correlated gaps at B = 8 T.
Fig. 3: Emergence of zero-field correlated gaps and symmetry-breaking cascade.
Fig. 4: Temperature dependence of correlated gaps around ν = ±2 and the symmetry-breaking cascade.

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.

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Acknowledgements

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.

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Author notes

  1. These authors contributed equally to this work: Youngjoon Choi, Hyunjin Kim.

Authors and Affiliations

  1. T. J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA, USA

    Youngjoon Choi, Hyunjin Kim, Robert Polski, Yiran Zhang & Stevan Nadj-Perge

  2. Institute for Quantum Information and Matter, California Institute of Technology, Pasadena, CA, USA

    Youngjoon Choi, Hyunjin Kim, Cyprian Lewandowski, Alex Thomson, Robert Polski, Yiran Zhang, Jason Alicea & Stevan Nadj-Perge

  3. Department of Physics, California Institute of Technology, Pasadena, CA, USA

    Youngjoon Choi, Hyunjin Kim, Cyprian Lewandowski, Alex Thomson, Yiran Zhang & Jason Alicea

  4. Walter Burke Institute for Theoretical Physics, California Institute of Technology, Pasadena, CA, USA

    Cyprian Lewandowski, Alex Thomson & Jason Alicea

  5. Department of Physics and Astronomy, California State University, Northridge, CA, USA

    Yang Peng

  6. National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe & Takashi Taniguchi

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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.

Corresponding author

Correspondence to Stevan Nadj-Perge.

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The authors declare no competing interests.

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Supplementary Discussion and Figs. 1–16.

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Choi, Y., Kim, H., Lewandowski, C. et al. Interaction-driven band flattening and correlated phases in twisted bilayer graphene. Nat. Phys. 17, 1375–1381 (2021). https://doi.org/10.1038/s41567-021-01359-0

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