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Visualization of the flat electronic band in twisted bilayer graphene near the magic angle twist

Nature Physics volume 17, pages 184–188 (2021)Cite this article

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Abstract

Bilayer graphene has been predicted to host a moiré miniband with flat dispersion if the layers are stacked at specific twist angles known as the ’magic angles’1,2. Recently, twisted bilayer graphene (tBLG) with a magic angle twist was reported to exhibit a correlated insulating state and superconductivity3,4, where the presence of the flat miniband in the system is thought to be essential for the emergence of these ordered phases in the transport measurements. Although tunnelling spectroscopy5,6,7,8,9 and electronic compressibility measurements10 in tBLG have found a van Hove singularity that is consistent with the presence of the flat miniband, a direct observation of the flat dispersion in the momentum space of such a moiré miniband in tBLG is still lacking. Here, we report the visualization of this flat moiré miniband by using angle-resolved photoemission spectroscopy with nanoscale resolution. The high spatial resolution of this technique enabled the measurement of the local electronic structure of the tBLG. The measurements demonstrate the existence of the flat moiré band near the charge neutrality for tBLG close to the magic angle at room temperature.

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Fig. 1: tBLG near magic angle twist on hBN substrate.
Fig. 2: Moiré pattern visualized with MIM from tBLG in the location with flat electronic band feature.
Fig. 3: Electronic structure of tBLG and the visualization of the flat band dispersion.

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Data availability

The data that support the plots within this paper and other finding of this study are available from the corresponding author upon reasonable request. Simulation parameters are provided in the Supplementary Information and can be used as in LAMMPS or with the KIM MD potential database.

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Acknowledgements

We thank S. Kahn for technical assistance in the sample fabrication setup. This work was supported as part of the Center for Novel Pathways to Quantum Coherence in Materials, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. The MIM measurements were supported by the Materials Sciences and Engineering Division of the US Department of Energy under contract no. DE-AC02-05-CH11231 (sp2-Bonded Materials Program KC2207). The MIM instrument at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. Support for K.W. and T.T. from the Elemental Strategy Initiative, conducted by the MEXT, Japan and CREST (JPMJCR15F3) is acknowledged. J.S.T. and N.L. acknowledge the Korean National Research Foundation grant no. NRF-2018R1C1B6004437 and the Korea Research Fellowship Program funded by the Ministry of Science and ICT (KRF-2016H1D3A1023826), as well as the computational resources by KISTI through grant no. KSC-2018-CHA-0077. J.J. was supported by the Samsung Science and Technology Foundation under project SSTF-BAA1802-06.

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Author notes

  1. These authors contributed equally: M. Iqbal Bakti Utama, Roland J. Koch and Kyunghoon Lee.

Authors and Affiliations

  1. Department of Physics, University of California at Berkeley, Berkeley, CA, USA

    M. Iqbal Bakti Utama, Kyunghoon Lee, Hongyuan Li, Sihan Zhao, Lili Jiang, Jiayi Zhu, Alex Zettl & Feng Wang

  2. Department of Materials Science and Engineering, University of California at Berkeley, Berkeley, CA, USA

    M. Iqbal Bakti Utama

  3. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    M. Iqbal Bakti Utama, Kyunghoon Lee, Alex Zettl & Feng Wang

  4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Roland J. Koch, Chris Jozwiak, Eli Rotenberg & Aaron Bostwick

  5. Department of Physics, University of Seoul, Seoul, South Korea

    Nicolas Leconte & Jeil Jung

  6. Graduate Group in Applied Science and Technology, University of California at Berkeley, Berkeley, CA, USA

    Hongyuan Li

  7. National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe & Takashi Taniguchi

  8. Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Paul D. Ashby & Alexander Weber-Bargioni

  9. Kavli Energy NanoSciences Institute, University of California at Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, CA, USA

    Alex Zettl & Feng Wang

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Contributions

F.W., E.R. and M.I.B.U. conceived the project. M.I.B.U. developed the sample preparation method and carried out sample fabrication with the assistance of J.Z. The nanoARPES experiments were performed by R.J.K., A.B. and E.R. The nanoARPES setup was developed and maintained by R.J.K., C.J., A.B. and E.R. The nanoARPES experimental data were analysed by M.I.B.U. and F.W. with A.B. and E.R. providing guidance. K.L. contributed to the MIM instrumentation setup. K.L. and M.I.B.U. performed AFM and MIM and analysed the data with P.D.A., A.W.B. and A.Z. providing guidance. J.J. and N.L. calculated the spectral functions. H.L. and S.Z. contributed to the surface cleaning process. L.J. performed scanning near field optical microscopy. K.W. and T.T. grew the hBN single crystal. F.W. and E.R. supervised the project. M.I.B.U. and F.W. wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to Jeil Jung, Aaron Bostwick or Feng Wang.

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Supplementary Information

Supplementary Figs. 1–9 with the accompanying discussion.

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Utama, M.I.B., Koch, R.J., Lee, K. et al. Visualization of the flat electronic band in twisted bilayer graphene near the magic angle twist. Nat. Phys. 17, 184–188 (2021). https://doi.org/10.1038/s41567-020-0974-x

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