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 (2D) crystal can be electrostatically tuned from a quantum spin Hall insulator (QSHI) to a superconductor. However, much remains unknown about the gap-opening mechanism of the insulating state. Here we report evidence that the QSHI is also an excitonic insulator (EI), arising from the spontaneous formation of electron-hole bound states (excitons). We reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples and confirm the correlated nature of this charge-neutral insulator by tunneling spectroscopy. We provide evidence against alternative scenarios of a band insulator or a localized insulator and support the existence of an EI 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.
@article{arxiv.2010.05390,
title = {Evidence for a Monolayer Excitonic Insulator},
author = {Yanyu Jia and Pengjie Wang and Cheng-Li Chiu and Zhida Song and Guo Yu and Berthold Jäck and Shiming Lei and Sebastian Klemenz and F. Alexandre Cevallos and Michael Onyszczak and Nadezhda Fishchenko and Xiaomeng Liu and Gelareh Farahi and Fang Xie and Yuanfeng Xu and Kenji Watanabe and Takashi Taniguchi and B. Andrei Bernevig and Robert J. Cava and Leslie M. Schoop and Ali Yazdani and Sanfeng Wu},
journal= {arXiv preprint arXiv:2010.05390},
year = {2022}
}