  While exciton relaxation in transition metal dichalcogenides (TMDs) has been
intensively studied, spatial exciton propagation has received only little
attention - in spite of being a key process for optoelectronics and having
already shown interesting unconventional behaviours (e.g. spatial halos). Here,
we study the spatiotemporal dynamics in TMDs and track the way of optically
excited excitons in time, momentum, and space. In particular, we investigate
the temperature-dependent exciton diffusion including the remarkable exciton
landscape constituted by bright and dark states. Based on a fully quantum
mechanical approach, we show at low temperatures an unexpected negative
transient diffusion. This phenomenon can be traced back to the existence of
dark exciton states in TMDs and is a result of an interplay between spatial
exciton diffusion and intervalley exciton-phonon scattering.
