Nonlinear diffusion of negatively charged excitons in monolayer WSe2

We investigate the diffusion process of negatively charged excitons (trions) in a WSe2 transition-metal dichalcogenide monolayer. We measure the time-resolved photoluminescence spatial profiles of these excitonic complexes. They exhibit nonlinear diffusion corresponding to an effective negative diffusion behavior. Specifi-cally, we examine the dynamics of the two negatively charged bright excitons (intervalley and intravalley trions) and of the dark trion. The time evolution allows us to identify the interplay of the different excitonic species: the trionic species appear after the neutral excitonic one. This is consistent with a bimolecular formation mechanism. Based on our experimental observations, we propose a phenomenological model suggesting the coexistence of two populations: a first one exhibiting a fast and efficient diffusion mechanism and a second one with a slower dynamics and a less efficient diffusion process. These two contributions could be, respectively, attributed to hot and cold trion populations.