Propagation of dark excitons in monolayer semiconductors at cryogenic temperatures

Two-dimensional transition metal dichalcogenides offer an excellent platform to study the propagation of tightly-bound exciton quasiparticles. While the exciton diffusion has been largely explored at ambient conditions, at cryogenic temperatures the rapid decay of bright excitons portrays a major limitation. We therefore take advantage of the long-lived reservoirs of dark states that emit via phonon-assisted processes and emerge as characteristic sidebands in the low-temperature luminescence spectra. This allows us to monitor the dynamics and transport properties of these dark states at cryogenic conditions in hBN-encapsulated WSe₂ monolayer using time- and spatially-resolved optical measurements. Already at 5 K we find highly efficient diffusion of dark excitons that is intrinsically limited by scattering with the bath of acoustic phonons. No influence of localization from residual disorder is detected. Interestingly, with increasing temperature we find a pronounced decrease of the diffusion coefficient. This deviates from the semi-classical expectation from independently determined scattering rates and challenges the established description of the exciton transport in monolayer materials.