Observation of flat bands in twisted bilayer graphene

Abstract

Transport experiments in twisted bilayer graphene have revealed multiple superconducting domes separated by correlated insulating states1,2,3,4,5. These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moiré superlattice as was predicted by band structure calculations6,7,8. Evidence for the existence of a flat band comes from local tunnelling spectroscopy9,10,11,12,13 and electronic compressibility measurements14, which report two or more sharp peaks in the density of states that may be associated with closely spaced Van Hove singularities. However, direct momentum-resolved measurements have proved to be challenging15. Here, we combine different imaging techniques and angle-resolved photoemission with simultaneous real- and momentum-space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with a homogeneous twist angle that support a flat band with a spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands, which show multiple moiré hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.

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Fig. 1: Device layout.
Fig. 2: Device characterization.
Fig. 3: ARPES spectral weight distribution.
Fig. 4: Flat band and hybridization gaps.

Data availability

Supporting data are available for this paper in ref. 35. All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

We thank J. Aarts, S. Nadj-Perge, A. Yazdani, A. Pasupathy, A. Morpurgo, I. Gutierrez-Lezama, H. Henck, F. Groenewoud, K. van Oosten, R.M. Tromp, R. Wijgman and H. Zandvliet for discussions. We thank M. Hesselberth for technical LEEM support. The ARPES work was supported by the Swiss National Science Foundation (SNSF) through grant 200020_184998. L.R. acknowledges support by the SNSF through an Ambizione grant. The STM work was supported by the European Research Council (ERC StG SpinMelt) and by the Dutch Research Council (NWO), as part of the Frontiers of Nanoscience programme, as well as through a Vidi grant (680-47-536). The LEEM work was supported by the NWO as part of the Frontiers of Nanoscience programme. Growth of hBN crystals was supported by the MEXT Element Strategy Initiative to Form Core Research Center (JPMXP0112101001) and the Core Research for Evolutional Science and Technology (JPMJCR15F3), Japan Science and Technology Agency. D.K.E. acknowledges support from the Ministry of Economy and Competitiveness of Spain through the Severo Ochoa programme for Centres of Excellence in R&D (SE5-0522), Funda-ció Privada Cellex, Fundació Privada Mir-Puig, the Generalitat de Catalunya through the CERCA programme, the H2020 Programme (820378), 2D·SIPC and the La Caixa Foundation.

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Contributions

X.L., P.S. and J.R.D. made the TBG devices. T.T. and K.W. contributed hBN materials. S.L., F.M., I.C., E.C. and A.H. performed the nano-ARPES experiments. T.B., V.S. and M.L. performed the STM experiments. T.A.d.J. acquired the LEEM and microscopic low-energy electron diffraction data. L.R. performed the band structure calculations. J.J., S.J.v.d.M. (LEEM), M.A. (STM), D.K.E. (devices) and F.B. (nano-ARPES) were responsible for the project supervision and the provision of resources. V.K., A.G. and A.B. were responsible for the nano-ARPES beamline. S.L., A.T. and F.B. wrote the bulk of the manuscript with contributions from several others. All authors contributed to the scientific discussion of the results.

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Correspondence to Felix Baumberger.

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The authors declare no competing interests.

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Peer review information Nature Physics thanks Zhongkai Liu and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Information

Supplementary Figs. 1–5 and Supplementary Discussion.

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Lisi, S., Lu, X., Benschop, T. et al. Observation of flat bands in twisted bilayer graphene. Nat. Phys. (2020). https://doi.org/10.1038/s41567-020-01041-x

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