Impact of free carriers on exciton and trion diffusion in monolayer WSe₂

Propagation of light-emitting exciton quasiparticles is of central importance in the fields of condensed matter physics and nanomaterials science. Monolayer transition metal dichalcogenides are particularly suitable to study this phenomenon due to strong Coulomb interaction and efficient light-matter coupling. In these systems the excitons are tightly bound, but also mobile at both room and cryogenic temperatures, as well as interact strongly with free charge carriers. The impact of the exciton-electron interaction in the context of exciton propagation, however, remains unclear, while there is increasingly mounting evidence of mobile composite exciton-carrier states, known as trions and Fermi polarons. Here we address this question in a controlled experimental scenario and demonstrate diffusion of excitons in the presence of a continuously tunable Fermi sea. Studying hBN-encapsulated WSe₂ monolayers by ultrafast microscopy we reveal a non-monotonic dependence of the exciton diffusion coefficient on the charge carrier density in both electron and hole doped regimes. We identify distinct regimes of elastic scattering and quasiparticle formation determining exciton diffusion and highlight the importance of treating exciton-electron scattering in the presence of additional energy and momentum dissipation via phonons. We further emphasize that both excitons and trions remain mobile down to 5 K, with an effective trion mobility reaching up to 3000 cm²/(Vs).