Evidence for a Monolayer Excitonic Insulator

TL;DR

This paper presents evidence that monolayer WTe2 exhibits an excitonic insulator phase, combining topological properties with electron-hole pairing, and provides experimental confirmation of this correlated insulating state.

Contribution

It demonstrates that the quantum spin Hall insulator in monolayer WTe2 is also an excitonic insulator, revealing a new correlated topological phase.

Findings

Intrinsic insulating state at charge neutrality point

Evidence against band or localized insulator scenarios

Confirmation of excitonic insulator phase in clean samples

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 (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.