  We investigate the diffusion process of negatively charged excitons (trions)
in WSe2 transition metal dichalcogenide monolayer. We measure time-resolved
photoluminescence spatial profiles of these excitonic complexes which exhibit a
non-linear diffusion process with an effective negative diffusion behavior.
Specifically, we examine the dynamics of the two negatively charged bright
excitons (intervalley and intravalley trion) as well as the dark trion. The
time evolution allows us to identify the interplay of different excitonic
species: the trionic species appear after the neutral excitonic ones,
consistent with a bimolecular formation mechanism. Using the 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 attributed to hot and cold
trion populations.
