Negative excitonic diffusion in transition metal dichalcogenides

TL;DR

This paper investigates the unusual negative exciton diffusion in transition metal dichalcogenides, revealing how dark exciton states and phonon interactions cause this counterintuitive behavior, with implications for optoelectronic applications.

Contribution

It introduces a quantum mechanical model to explain negative exciton diffusion in TMDs, highlighting the role of dark states and intervalley scattering at low temperatures.

Findings

Negative transient exciton diffusion observed at low temperatures.

Dark exciton states influence spatial exciton dynamics.

Intervalley exciton-phonon scattering affects diffusion behavior.

Abstract

While exciton relaxation in transition metal dichalcogenides (TMDs) has been intensively studied, spatial exciton propagation has received only little attention - in spite of being a key process for optoelectronics and having already shown interesting unconventional behaviours (e.g. spatial halos). Here, we study the spatiotemporal dynamics in TMDs and track the way of optically excited excitons in time, momentum, and space. In particular, we investigate the temperature-dependent exciton diffusion including the remarkable exciton landscape constituted by bright and dark states. Based on a fully quantum mechanical approach, we show at low temperatures an unexpected negative transient diffusion. This phenomenon can be traced back to the existence of dark exciton states in TMDs and is a result of an interplay between spatial exciton diffusion and intervalley exciton-phonon scattering.