Evidence for a monolayer excitonic insulator

Journal Article · 22 December 2021 · Nature Physics

DOI: https://doi.org/10.1038/s41567-021-01422-w · OSTI ID:1842827

Jia, Yanyu  ^[1]; Wang, Pengjie  ^[2]; Chiu, Cheng-Li  ^[2]; Song, Zhida  ^[2]; Yu, Guo  ^[2]; Jäck, Berthold ^[2]; Lei, Shiming ^[2]; Klemenz, Sebastian ^[2]; Cevallos, F. Alexandre ^[2]; Onyszczak, Michael  ^[2]; Fishchenko, Nadezhda ^[2]; Liu, Xiaomeng  ^[2]; Farahi, Gelareh ^[2]; Xie, Fang ^[2]; Xu, Yuanfeng  ^[3]; Watanabe, Kenji  ^[4]; Taniguchi, Takashi  ^[4]; Bernevig, B. Andrei ^[2]; Cava, Robert J. ^[2]; Schoop, Leslie M.  ^[2] ; Yazdani, Ali  ^[2]; Wu, Sanfeng  ^[2]

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1. Princeton Univ., NJ (United States); Princeton University
2. Princeton Univ., NJ (United States)
3. Max Planck Inst. for Microstructure Physics, Halle (Germany)
4. National Institute for Materials Science (NIMS), Tsukuba (Japan)

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. In this work, 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.

• Research Organization: Princeton Univ., NJ (United States)
• Sponsoring Organization: Gordon and Betty Moore Foundation; Japan Society for the Promotion of Science (JSPS); National Science Foundation (NSF); Simons Foundation; US Army Research Office (ARO); US Department of the Navy, Office of Naval Research (ONR); USDOE Office of Science (SC), Basic Energy Sciences (BES); United States – Israel Binational Science Foundation (BSF)
• Grant/Contract Number: FG02-07ER46419; SC0016239
• OSTI ID: 1842827
• Journal Information: Nature Physics, Journal Name: Nature Physics Journal Issue: 1 Vol. 18; ISSN 1745-2473
• Publisher: Nature Publishing Group (NPG)Copyright Statement
• Country of Publication: United States
• Language: English