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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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Fig. 1: Possible scenarios of the ground states at the charge neutrality point in monolayer WTe2.
Fig. 2: The insulating state at charge neutrality in monolayer WTe2.
Fig. 3: Hall anomaly in the monolayer insulator.
Fig. 4: Signature of correlations and the metal–insulator transition revealed by tunnelling spectroscopy.

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.

References

  1. Ren, Y., Qiao, Z. & Niu, Q. Topological phases in two-dimensional materials: a review. Rep. Prog. Phys. 79, 066501 (2016).

    ADS  Google Scholar 

  2. Jérome, D., Rice, T. M. & Kohn, W. Excitonic insulator. Phys. Rev. 158, 462–475 (1967).

    ADS  Google Scholar 

  3. Kohn, W. Excitonic phases. Phys. Rev. Lett. 19, 439–442 (1967).

    ADS  Google Scholar 

  4. Blatt, J. M., Böer, K. W. & Brandt, W. Bose-Einstein condensation of excitons. Phys. Rev. 126, 1691–1692 (1962).

    ADS  Google Scholar 

  5. Kotov, V. N., Uchoa, B., Pereira, V. M., Guinea, F. & Castro Neto, A. H. Electron-electron interactions in graphene: current status and perspectives. Rev. Mod. Phys. 84, 1067–1125 (2012).

    ADS  Google Scholar 

  6. Elias, D. C. et al. Dirac cones reshaped by interaction effects in suspended graphene. Nat. Phys. 7, 701–704 (2011).

    Google Scholar 

  7. Varsano, D., Palummo, M., Molinari, E. & Rontani, M. A monolayer transition-metal dichalcogenide as a topological excitonic insulator. Nat. Nanotechnol. 15, 367–372 (2020).

    ADS  Google Scholar 

  8. Zheng, B. & Fu, L. Excitonic density wave and spin-valley superfluid in bilayer transition metal dichalcogenide. Nat. Commun. 12, 642 (2021).

    Google Scholar 

  9. Barkeshli, M., Nayak, C., Papić, Z., Young, A. & Zaletel, M. Topological exciton Fermi surfaces in two-component fractional quantized Hall insulators. Phys. Rev. Lett. 121, 026603 (2018).

    ADS  Google Scholar 

  10. Pikulin, D. I. & Hyart, T. Interplay of exciton condensation and the quantum spin Hall effect in InAs/GaSb bilayers. Phys. Rev. Lett. 112, 176403 (2014).

    ADS  Google Scholar 

  11. Blason, A. & Fabrizio, M. Exciton topology and condensation in a model quantum spin Hall insulator. Phys. Rev. B 102, 035146 (2020).

    ADS  Google Scholar 

  12. Hu, Y., Venderbos, J. W. F. & Kane, C. L. Fractional excitonic insulator. Phys. Rev. Lett. 121, 126601 (2018).

    ADS  Google Scholar 

  13. Chowdhury, D., Sodemann, I. & Senthil, T. Mixed-valence insulators with neutral Fermi surfaces. Nat. Commun. 9, 1766 (2018).

    ADS  Google Scholar 

  14. Cercellier, H. et al. Evidence for an excitonic insulator phase in 1T–TiSe2. Phys. Rev. Lett. 99, 146403 (2007).

    ADS  Google Scholar 

  15. Li, Z. et al. Possible excitonic insulating phase in quantum-confined Sb nanoflakes. Nano Lett. 19, 4960–4964 (2019).

    ADS  Google Scholar 

  16. Kogar, A. et al. Signatures of exciton condensation in a transition metal dichalcogenide. Science 358, 1314–1317 (2017).

    ADS  Google Scholar 

  17. Du, L. et al. Evidence for a topological excitonic insulator in InAs/GaSb bilayers. Nat. Commun. 8, 1971 (2017).

    ADS  Google Scholar 

  18. Wakisaka, Y. et al. Excitonic insulator state in Ta2NiSe5 probed by photoemission spectroscopy. Phys. Rev. Lett. 103, 026402 (2009).

    ADS  Google Scholar 

  19. Lu, Y. F. et al. Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5. Nat. Commun. 8, 14408 (2017).

    ADS  Google Scholar 

  20. Fukutani, K. et al. Electrical tuning of the excitonic insulator ground state of Ta2NiSe5. Phys. Rev. Lett. 123, 206401 (2019).

    ADS  Google Scholar 

  21. Yu, W. et al. Anomalously large resistance at the charge neutrality point in a zero-gap InAs/GaSb bilayer. New J. Phys. 20, 053062 (2018).

    ADS  Google Scholar 

  22. Eisenstein, J. P. Exciton condensation in bilayer quantum Hall systems. Annu. Rev. Condens. Matter Phys. 5, 159–181 (2014).

    ADS  Google Scholar 

  23. Liu, X., Watanabe, K., Taniguchi, T., Halperin, B. I. & Kim, P. Quantum Hall drag of exciton condensate in graphene. Nat. Phys. 13, 746–750 (2017).

    Google Scholar 

  24. Li, J. I. A., Taniguchi, T., Watanabe, K., Hone, J. & Dean, C. R. Excitonic superfluid phase in double bilayer graphene. Nat. Phys. 13, 751–755 (2017).

    Google Scholar 

  25. Qian, X., Liu, J., Fu, L. & Li, J. Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. Science 346, 1344–1347 (2014).

    ADS  Google Scholar 

  26. Fei, Z. et al. Edge conduction in monolayer WTe2. Nat. Phys. 13, 677–682 (2017).

    Google Scholar 

  27. Tang, S. et al. Quantum spin Hall state in monolayer 1T′-WTe2. Nat. Phys. 13, 683–687 (2017).

    Google Scholar 

  28. Wu, S. et al. Observation of the quantum spin Hall effect up to 100 kelvin in a monolayer crystal. Science 359, 76–79 (2018).

    ADS  MathSciNet  MATH  Google Scholar 

  29. Fatemi, V. et al. Electrically tunable low-density superconductivity in a monolayer topological insulator. Science 362, 926–929 (2018).

    ADS  MathSciNet  Google Scholar 

  30. Sajadi, E. et al. Gate-induced superconductivity in a monolayer topological insulator. Science 362, 922–925 (2018).

    ADS  Google Scholar 

  31. Muechler, L., Alexandradinata, A., Neupert, T. & Car, R. Topological nonsymmorphic metals from band inversion. Phys. Rev. X 6, 041069 (2016).

    Google Scholar 

  32. Zheng, F. et al. On the quantum spin Hall gap of monolayer 1T′-WTe2. Adv. Mater. 28, 4845–4851 (2016).

    Google Scholar 

  33. Xu, S.-Y. et al. Electrically switchable Berry curvature dipole in the monolayer topological insulator WTe2. Nat. Phys. 14, 900–906 (2018).

    Google Scholar 

  34. Song, Y.-H. et al. Observation of Coulomb gap in the quantum spin Hall candidate single-layer 1T′-WTe2. Nat. Commun. 9, 4071 (2018).

    ADS  Google Scholar 

  35. Rhodes, D., Chae, S. H., Ribeiro-Palau, R. & Hone, J. Disorder in van der Waals heterostructures of 2D materials. Nat. Mater. 18, 541–549 (2019).

    ADS  Google Scholar 

  36. Ali, M. N. et al. Correlation of crystal quality and extreme magnetoresistance of WTe2. Europhys. Lett. 110, 67002 (2015).

    ADS  Google Scholar 

  37. Efros, A. L. & Shklovskii, B. I. Coulomb gap and low temperature conductivity of disordered systems. J. Phys. C 8, L49–L51 (1975).

    ADS  Google Scholar 

  38. Kivelson, S., Lee, D.-H. & Zhang, S.-C. Global phase diagram in the quantum Hall effect. Phys. Rev. B 46, 2223–2238 (1992).

    ADS  Google Scholar 

  39. Hilke, M. et al. Experimental evidence for a two-dimensional quantized Hall insulator. Nature 395, 675–677 (1998).

    ADS  Google Scholar 

  40. Ebisawa, H. & Fukuyama, H. Hall effect in excitonic insulator. Prog. Theor. Phys. 42, 512–522 (1969).

    ADS  Google Scholar 

  41. Campbell, D. J. et al. Intrinsic insulating ground state in transition metal dichalcogenide TiSe2. Phys. Rev. Mater. 3, 053402 (2019).

    Google Scholar 

  42. Li, G. et al. Semimetal-to-semimetal charge density wave transition in 1T−TiSe2. Phys. Rev. Lett. 99, 027404 (2007).

    ADS  Google Scholar 

  43. Ponomarenko, L. A. et al. Cloning of Dirac fermions in graphene superlattices. Nature 497, 594–597 (2013).

    ADS  Google Scholar 

  44. Cao, Y. et al. Tunable correlated states and spin-polarized phases in twisted bilayer–bilayer graphene. Nature 583, 215–220 (2020).

    ADS  Google Scholar 

  45. Chandni, U., Watanabe, K., Taniguchi, T. & Eisenstein, J. P. Signatures of phonon and defect-assisted tunneling in planar metal–hexagonal boron nitride–graphene junctions. Nano Lett. 16, 7982–7987 (2016).

    ADS  Google Scholar 

  46. Raja, A. et al. Coulomb engineering of the bandgap and excitons in two-dimensional materials. Nat. Commun. 8, 15251 (2017).

    ADS  Google Scholar 

  47. Zhao, C. et al. Strain tunable semimetal–topological-insulator transition in monolayer 1T′–WTe2. Phys. Rev. Lett. 125, 046801 (2020).

    ADS  Google Scholar 

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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.

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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.

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Correspondence to Ali Yazdani or Sanfeng Wu.

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The authors declare no competing interests.

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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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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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