Ultrafast exciton fluid flow in an atomically thin MoS 2 semiconductor.

Excitons (coupled electron-hole pairs) in semiconductors can form collective states that sometimes exhibit spectacular nonlinear properties. Here, we show experimental evidence of a collective state of short-lived excitons in a direct-bandgap, atomically thin MoS 2 semiconductor whose propagation resembles that of a classical liquid as suggested by the nearly uniform photoluminescence through the MoS 2 monolayer regardless of crystallographic defects and geometric constraints. The exciton fluid flows over ultralong distances (at least 60 μm) at a speed of ~1.8 × 10 7  m s -1 (~6% the speed of light). The collective phase emerges above a critical laser power, in the absence of free charges and below a critical temperature (usually T c  ≈ 150 K) approaching room temperature in hexagonal-boron-nitride-encapsulated devices. Our theoretical simulations suggest that momentum is conserved and local equilibrium is achieved among excitons; both these features are compatible with a fluid dynamics description of the exciton transport.