Evidence for a Monolayer Excitonic Insulator

The excitonic insulator (EI) phase, arising from the spontaneous formation of electron-hole bound states in semimetals or small gap semiconductors, has been predicted for decades but its experimental observation remains elusive. In this talk, we report systematic studies of WTe₂ monolayer insulator and uncover evidence supporting this monolayer crystal as an EI. Our experiments on high-quality transport devices reveal the presence of an intrinsic insulating state at the charge neutrality point (CNP) in clean samples. The state exhibits both a strong sensitivity to the electric displacement field and a Hall anomaly that are consistent with the excitonic pairing. We confirm the correlated nature of the charge-neutral insulator by tunneling spectroscopy. The results support the existence of an EI phase in the clean limit and rule out alternative scenarios of a band insulator or a localized insulator. 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.