Diffusion of excitons immersed in the Fermi sea of free charges in monolayer semiconductors*

Interactions of excitons with free charge carriers represents an intriguing area of research in semiconductor physics. In monolayer transition metal dichalcogenides tightly bound excitons are mobile at both room temperature and crypgenic temperatures and strongly interact with free electrons and holes. Here we employ transient microscopy in a hBN-encapsulated monolayer WSe₂ field-effect transistor studying diffusion of long-lived dark exciton states in the presence of free charge carriers. We find a non-monotonous behavior in the diffusivity revealing the strong impact of free electron and hole scattering in the perturbative doping regime and investigate trion and Fermi-polaron propagation at elevated charge carrier densities. Our study opens up avenues to investigate transport of Fermi-Bose quasiparticle mixtures and offers insight into the interactions of mobile excitons with free charge carriers.