Correlated insulator of excitons in WSe2/WS2 moir\'e superlattices

A panoply of unconventional electronic states has been observed in moir\'e superlattices. Engineering similar bosonic phases remains, however, largely unexplored. We report the observation of a bosonic correlated insulator in WSe2/WS2 moir\'e superlattices composed of excitons, i.e., tightly bound electron-hole pairs. We develop a pump probe spectroscopy method that we use to observe an exciton incompressible state at exciton filling {\nu}_ex = 1 and charge neutrality, indicating a correlated insulator of excitons. With varying charge density, the bosonic correlated insulator continuously transitions into an electron correlated insulator at charge filling {\nu}_e = 1, suggesting a mixed correlated insulating state between the two limits. Our studies establish semiconducting moir\'e superlattices as an intriguing platform for engineering bosonic phases.