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.
@article{arxiv.2107.11168,
title = {Nonclassical Exciton Diffusion in Monolayer WSe2},
author = {Koloman Wagner and Jonas Zipfel and Roberto Rosati and Edith Wietek and Jonas D. Ziegler and Samuel Brem and Raül Pera-Causín and Takashi Taniguchi and Kenji Watanabe and Mikhail M. Glazov and Ermin Malic and Alexey Chernikov},
journal= {arXiv preprint arXiv:2107.11168},
year = {2021}
}