Nonclassical Exciton Diffusion in Monolayer WSe2

TL;DR

This study reveals unusual nonclassical exciton diffusion behavior in monolayer WSe2 at cryogenic temperatures, challenging traditional models and opening new avenues for quantum transport research in 2D materials.

Contribution

It provides the first experimental evidence of nonclassical exciton diffusion in monolayer WSe2 and introduces models explaining this novel behavior.

Findings

Diffusivity decreases with temperature below phonon activation threshold.

Diffusion behavior deviates from classical free-particle and hopping models.

Experimental observation of phonon-assisted dark state recombination.

Abstract

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.