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# Evidence for a monolayer excitonic insulator

## Abstract

The interplay between topology and correlations can generate a variety of quantum phases, many of which remain to be explored. Recent advances have identified monolayer WTe2 as a promising material for doing so in a highly tunable fashion. The ground state of this two-dimensional crystal can be electrostatically tuned from a quantum spin Hall insulator to a superconductor. However, much remains unknown about the gap-opening mechanism of the insulating state. Here we report evidence that the quantum spin Hall insulator is also an excitonic insulator, arising from the spontaneous formation of electron–hole bound states, namely excitons. We reveal the presence of an intrinsic insulating state at the charge neutrality point in clean samples and confirm the correlated nature of this charge-neutral insulator by tunnelling spectroscopy. We provide evidence against alternative scenarios of a band insulator or a localized insulator and support the existence of an excitonic insulator phase in the clean limit. These observations lay the foundation for understanding a new class of correlated insulators with nontrivial topology and identify monolayer WTe2 as a promising candidate for exploring quantum phases of ground-state excitons.

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## Data availability

The data that support the plots within this paper are available at https://doi.org/10.7910/DVN/FFGQOX. Other data that support the findings of this study are available from the corresponding authors upon reasonable request.

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## Acknowledgements

We acknowledge helpful discussions with N. P. Ong and P. A. Lee. Work in the Wu lab was primarily supported by the National Science Foundation (NSF) through a CAREER award to S.W. (DMR-1942942). Device fabrication was supported by NSF-MRSEC through the Princeton Center for Complex Materials (DMR-1420541 and DMR-2011750). S.W. and L.M.S. acknowledge the support from the Eric and Wendy Schmidt Transformative Technology Fund at Princeton. Part of the measurements was performed at the National High Magnetic Field Laboratory, which is supported by NSF cooperative agreement no. DMR-1644779 and the State of Florida. Work in the Yazdani lab was primarily supported by the Gordon and Betty Moore Foundation EPiQS initiative grants GBMF4530 and GBMF9469 and by the Department of Energy (DOE) BES grant DE-FG02-07ER46419. Other support for the experimental work by A.Y. was provided by NSF (DMR-1904442), ExxonMobil through the Andlinger Center for Energy and the Environment at Princeton, and the Princeton Catalysis Initiative. B.A.B. is supported by DOE grant no. DE-SC0016239, the Schmidt Fund for Innovative Research, Simons Investigator grant no. 404513 and the Packard Foundation for the numerical work. The analytical part was supported by NSF EAGER grant no. DMR-1643312, United States–Israel BSF grant no. 2018226, ONR grant no. N00014-20-1-2303 and the Princeton Global Network Funds. Additional support to B.A.B. was provided by the Gordon and Betty Moore Foundation through grant no. GBMF8685 towards the Princeton theory program. B.J. acknowledges funding through a postdoctoral fellowship of the Alexander-von-Humboldt Foundation. K.W. and T.T. acknowledge support from MEXT Element Strategy Initiative (Japan) grant no. JPMXP0112101001, JSPS KAKENHI grant no. JP20H00354 and the JST CREST (JPMJCR15F3). F.A.C. and R.J.C. acknowledge support from the ARO MURI on Topological Insulators (grant no. W911NF1210461). S.L, S.K. and L.M.S. acknowledge support from the Gordon and Betty Moore Foundation through grant no. GBMF9064 awarded to L.M.S.

## Author information

Authors

### Contributions

S.W. supervised transport and vdW tunnelling studies. A.Y. supervised STM studies. P.W. and G.Y. fabricated transport devices. Y.J. fabricated the vdW tunnelling devices, assisted by P.W., G.Y., M.O., N.F. and B.J. Y.J., P.W., and S.W. performed transport and vdW tunnelling measurements and analysed data. C.-L.C., Y.J., P.W. and X.L. fabricated the STM device. C.-L.C., G.F., X.L. and B.J. performed STM measurements and analysed data. Z.S., F.X., Y.X. and B.A.B. provided theoretical support. S.L., S.K., L.M.S., F.A.C. and R.J.C. grew and characterized bulk WTe2 crystals. K.W. and T.T. provided hBN crystals. All authors discussed the result and contributed to the writing of the paper.

### Corresponding authors

Correspondence to Ali Yazdani or Sanfeng Wu.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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

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## Supplementary information

### Supplementary Information

Supplementary Sections 1–14.

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Jia, Y., Wang, P., Chiu, CL. et al. Evidence for a monolayer excitonic insulator. Nat. Phys. 18, 87–93 (2022). https://doi.org/10.1038/s41567-021-01422-w

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• DOI: https://doi.org/10.1038/s41567-021-01422-w

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