  Atomically thin semiconductors provide an excellent platform to study
intriguing many-particle physics of tightly-bound excitons. In particular, the
properties of tungsten-based transition metal dichalcogenides are determined by
a complex manifold of bright and dark exciton states. While dark excitons are
known to dominate the relaxation dynamics and low-temperature
photoluminescence, their impact on the spatial propagation of excitons has
remained elusive. In our joint theory-experiment study, we address this
intriguing regime of dark state transport by resolving the spatio-temporal
exciton dynamics in hBN-encapsulated WSe$_2$ monolayers after resonant
excitation. We find clear evidence of an unconventional, time-dependent
diffusion during the first tens of picoseconds, exhibiting strong deviation
from the steady-state propagation. Dark exciton states are initially populated
by phonon emission from the bright states, resulting in creation of hot
excitons whose rapid expansion leads to a transient increase of the diffusion
coefficient by more than one order of magnitude. These findings are relevant
for both fundamental understanding of the spatio-temporal exciton dynamics in
atomically thin materials as well as their technological application by
enabling rapid diffusion.
