  We experimentally demonstrate time-resolved exciton propagation in a
monolayer semiconductor at cryogenic temperatures. Monitoring phonon-assisted
recombination of dark states, we find a highly unusual case of exciton
diffusion. While at 5 K the diffusivity is intrinsically limited by acoustic
phonon scattering, we observe a pronounced decrease of the diffusion
coefficient with increasing temperature, far below the activation threshold of
higher-energy phonon modes. This behavior corresponds neither to well-known
regimes of semiclassical free-particle transport nor to the thermally activated
hopping in systems with strong localization. Its origin is discussed in the
framework of both microscopic numerical and semi-phenomenological analytical
models illustrating the observed characteristics of nonclassical propagation.
Challenging the established description of mobile excitons in monolayer
semiconductors, these results open up avenues to study quantum transport
phenomena for excitonic quasiparticles in atomically-thin van der Waals
materials and their heterostructures.
