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Dynamically tunable moiré exciton Rydberg states in a monolayer semiconductor on twisted bilayer graphene

Nature Materials volume 23, pages 224–229 (2024)Cite this article

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Abstract

Moiré excitons are emergent optical excitations in two-dimensional semiconductors with moiré superlattice potentials. Although these excitations have been observed on several platforms, a system with dynamically tunable moiré potential to tailor their properties is yet to be realized. Here we present a continuously tunable moiré potential in monolayer WSe2, enabled by its proximity to twisted bilayer graphene (TBG) near the magic angle. By tuning local charge density via gating, TBG provides a spatially varying and dynamically tunable dielectric superlattice for modulation of monolayer WSe2 exciton wave functions. We observed emergent moiré exciton Rydberg branches with increased energy splitting following doping of TBG due to exciton wave function hybridization between bright and dark Rydberg states. In addition, emergent Rydberg states can probe strongly correlated states in TBG at the magic angle. Our study provides a new platform for engineering moiré excitons and optical accessibility to electronic states with small correlation gaps in TBG.

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Fig. 1: Localized charge distribution in TBG and its periodic screening of adjacent WSe2.
Fig. 2: Moiré exciton Rydberg states in monolayer WSe2 on TBG.
Fig. 3: Tunable hybridization of exciton Rydberg states.
Fig. 4: Exciton Rydberg state sensing of strongly correlated states in magic-angle TBG.

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Data availability

Source data are provided with this paper. All other data are available from the corresponding author on reasonable request.

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Codes used for data analysis in this study are also available from the corresponding author on reasonable request.

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Acknowledgements

We thank J. L. Li and H. Yu for helpful discussions. This work was supported mainly by the US Department of Energy Basic Energy Sciences under award no. DE-SC0018171 (to X.X., M.H. and J.C.). Sample fabrication was partially supported by the ARO MURI programme (grant no. W911NF-18-1-0431 to M.H.). Electrical transport measurement was partially supported by the US National Science Foundation through the UW Molecular Engineering Materials Center, a Materials Research Science and Engineering Center (no. DMR-1719797 to X.X. and M.Y.). STM/spectroscopy measurement is supported by the Center on Programmable Quantum Materials, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0019443 (to A.P. and E.S.). Work at the University of Hong Kong is supported by the Research Grants Council of Hong Kong SAR (nos. AoE/P-701/20 and HKU SRFS2122-7S05 to W.Y. and H.Z.). W.Y. also acknowledges support by the New Cornerstone Science Foundation. K.W. and T.T. acknowledge support from JSPS KAKENHI (grant nos. 19H05790, 20H00354 and 21H05233). X.X. acknowledges support from the State of Washington-funded Clean Energy Institute and from the Boeing Distinguished Professorship in Physics.

Author information

Author notes

  1. These authors contributed equally: Minhao He, Jiaqi Cai, Huiyuan Zheng.

Authors and Affiliations

  1. Department of Physics, University of Washington, Seattle, WA, USA

    Minhao He, Jiaqi Cai, Matthew Yankowitz & Xiaodong Xu

  2. Department of Physics, University of Hong Kong, Hong Kong, China

    Huiyuan Zheng & Wang Yao

  3. Department of Physics, Columbia University, New York, NY, USA

    Eric Seewald & Abhay Pasupathy

  4. International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Japan

    Takashi Taniguchi

  5. Research Center for Functional Materials, National Institute for Materials Science, Tsukuba, Japan

    Kenji Watanabe

  6. Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA

    Jiaqiang Yan

  7. Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN, USA

    Jiaqiang Yan

  8. Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA

    Matthew Yankowitz & Xiaodong Xu

  9. HKU-UCAS Joint Institute of Theoretical and Computational Physics at Hong Kong, Hong Kong, China

    Wang Yao

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Contributions

M.H. and J.C. performed transport and optical reflection measurements, under the supervision of X.X. and M.Y. M.H. fabricated samples. M.H., J.C., M.Y., W.Y. and X.X. analysed and interpreted results. H.Z. and W.Y. performed theoretical calculations. E.S. and A.P. performed STM measurements and analysed results. J.Y. synthesized and characterized bulk WSe2 crystals. T.T. and K.W. synthesized h-BN crystals. M.H., X.X., W.Y., J.C. and H.Z. wrote the paper with input from all authors. All authors discussed the results.

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Correspondence to Wang Yao or Xiaodong Xu.

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Nature Materials thanks Olga Kazakova and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Supplementary Notes 1–4 and Figs. 1–11.

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He, M., Cai, J., Zheng, H. et al. Dynamically tunable moiré exciton Rydberg states in a monolayer semiconductor on twisted bilayer graphene. Nat. Mater. 23, 224–229 (2024). https://doi.org/10.1038/s41563-023-01713-y

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