Twisted bilayer graphene with a twist angle of around 1.1° features a pair of isolated flat electronic bands and forms a platform for investigating strongly correlated electrons. Here, we use scanning tunnelling microscopy to probe the local properties of highly tunable twisted bilayer graphene devices and show that the flat bands deform when aligned with the Fermi level. When the bands are half-filled, we observe the development of gaps originating from correlated insulating states. Near charge neutrality, we find a previously unidentified correlated regime featuring an enhanced splitting of the flat bands. We describe this within a microscopic model that predicts a strong tendency towards nematic ordering. Our results provide insights into symmetry-breaking correlation effects and highlight the importance of electronic interactions for all filling fractions in twisted bilayer graphene.
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The experimental data and analyses that support the plots within this paper and the findings of this study are available from the corresponding author upon reasonable request.
The computer codes that support the plots within this paper and the findings of this study are available from the corresponding author upon reasonable request
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We gratefully acknowledge discussions with R. C. Ashoori, P. Jarillo-Herrero, A. Vishwanath, J. Eisenstein, A. Young and H. Beidenkopf. The STM work is in part supported by NSF DMR-1744011. Sample fabrication efforts are supported by the NSF through program NSF CAREER DMR-1753306. S.N.-P. acknowledges support from a KNI-Weathley fellowship. J.A., G.R., F.v.O., S.N.-P. and H.R. acknowledge the support of IQIM (NSF funded physics frontiers center). J.K. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG 406557161), Y.C. a Kwanjeong fellowship, F.v.O. DFG support through CRC 183, and J.A. support from the NSF through grant DMR-1723367. Y.P., A.T. and J.A. are grateful for support from the Walter Burke Institute for Theoretical Physics at Caltech.
The authors declare no competing interests.
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Additional technical and theoretical details, Supplementary Figs. 1–14, Table 1 and refs. 1–16.
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Nature Physics (2019)