Direct visualization of dark exciton condensates in moiré superlattices

Abstract Moiré superlattices have emerged as an unprecedented manipulation tool for engineering correlated quantum phenomena in van der Waals heterostructures1-4. With moiré potentials as a naturally configurable solid-state that sustains high exciton density, interlayer excitons in transition metal dichalcogenide (TMDC) heterostructures are expected to achieve high-temperature exciton condensation and related superfluidity5. However, the exciton condensation is usually optically inactive due to the finite momentum of interlayer excitons. The experimental observation of dark exciton condensation in moiré potentials remains challenging with traditional optical techniques. Here we directly visualize the dark-exciton condensation in twisted TMDC heterostructures using femtosecond transient absorption microscopy. We observe a quantum transition from classical exciton gas to exciton condensation by imaging temperature-dependent exciton transport. With decreasing temperature, exciton diffusion rates exhibit an accelerating downwards trend under a critical degeneracy temperature, indicative of exciton condensation. This result is attributed to moiré potentials drastically suppressing exciton transport to promote exciton condensation. The ability to image exciton condensates opens the door to quantum information processing6 and high-precision metrology in moiré superlattices.