Excitonic interaction-driven transport and dynamics of dark states in 2D semiconductors

Excitonic quantum fluid in transition metal dichalcogenides (TMDs) has recently emerged as a promising research focus for studying many-body physics and applications for efficient and tunable excitonic devices. The multivalley band structure and large spin-orbit coupling in TMDs result in an abundance of optically bright and dark excitonic states with different spin/valley configurations. In particular, the dark excitons have a permanent out-of-plane dipole, large binding energy, and a long lifetime. Therefore, they are promising candidates for long-range transport driven by exciton-exciton interaction. Here, we show that both direct and exchange Coulomb excitonic interactions are repulsive and they enhance the dark exciton transport. By employing a high-resolution spatially resolved PL setup in an encapsulated monolayer of WS2, we demonstrate that dark exciton can diffuse up to several micrometers. Furthermore, we conduct experiments in the engineered exciton landscape and show that the repulsive interaction can provide additional energy to dark exciton to propagate in an uphill energy landscape. This repulsion-driven long-range transport of dark states provides a route for excitonic devices that could be used for both classical and quantum information processing.