Exciton Condensation in an Atomically-thin MoS2 Semiconductor

Abstract Condensation of a dilute Bose gas of excitons (coupled electron-hole pairs) in a direct bandgap semiconductor was first theoretically predicted in 19681. This exotic state of matter is expected to exhibit spectacular non-linear properties, such as superradiance and superfluidity. However, direct experimental observation of condensation of optically active excitons in conventional semiconductors has been hindered by their short lifetimes and weak collective excitonic interactions. Here, we have experimentally realized the condensation of short-lived excitons in a direct-bandgap, atomically-thin MoS2 semiconductor. The signature is the anomalous transport of the fast-expanding exciton density, originating from a thermalized dilute gas generated under the laser spot. Below the critical temperature Tc~150 K, the exciton liquid propagates over ultra-long distances (at least 60 micrometers) with record speed in a solid-state system of 1.8*10^7 m/s (~6% the speed of light), fuelled by the unconventionally strong repulsions among excitons. The condensation is controlled by many-body interactions in the gas mixture of excitons (bosons) and free-carriers (fermions) via an electrical backgate. Our results demonstrate electrostatic doping as a simple approach for the investigation of correlated states of matter at high-temperatures, excitonic circuitry and spin-valley Hall devices mediated by exciton superfluids in semiconducting monolayers.