上海科技大学知识管理系统

The excitonic insulator (EI) is a Bose-Einstein condensation (BEC) of excitons bound by electron-hole interaction in a solid, which could support high temperature BEC transition1-3. The material realization of EI has been elusive, which is further challenged by the difficulty of distinguishing it from a conventional charge density wave (CDW) state. In the BEC limit, the pre condensation exciton gas phase4 is a hallmark to distinguish EI from conventional CDW, yet direct experimental evidence has been lacking5. Here we report a distinct correlated phase beyond the 2×2 CDW ground state emerging in epitaxially grown monolayer 1T-ZrTe2 and its investigation by angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM). The results show novel band-and energy-dependent folding behavior in a two-step process, evidenced by an exciton gas phase prior to its condensation into the final CDW state. The excellent agreement between experiments and theoretical predictions on the recovery of the pristine band structure by carrier-density dependent suppression of the CDW state further corroborates the monolayer 1TZrTe2 as an EI. Our findings provide a versatile two-dimensional platform that allows tuning of the excitonic effect.