Non-linear and negative effective diffusivity of optical excitations in moir\'e-free heterobilayers

TL;DR

This study reveals that interlayer excitons in MoSe2/WSe2 heterobilayers exhibit highly efficient, non-linear diffusion driven by exciton interactions, with implications for optoelectronic applications and understanding exciton dynamics.

Contribution

It demonstrates nearly 1000-fold higher diffusion coefficients for interlayer excitons and disentangles exciton-exciton interactions from trapping effects in moiré-free heterobilayers.

Findings

Interlayer excitons show diffusion coefficients nearly 1000 times higher than previous reports.

Exciton-exciton repulsion and annihilation both significantly influence non-linear propagation.

Transient emission shrinking indicates a transition from exciton to plasma regimes.

Abstract

Interlayer exciton diffusion is studied in atomically-reconstructed MoSe2/WSe2 heterobilayers with suppressed disorder. Local atomic registry is confirmed by characteristic optical absorption, circularly-polarized photoluminescence, and g-factor measurements. Using transient microscopy we observe propagation properties of interlayer excitons that are independent from trapping at moir\'e- or disorder-induced local potentials. Confirmed by characteristic temperature dependence for free particles, linear diffusion coefficients of interlayer excitons at liquid helium temperature and low excitation densities are almost 1000 times higher than in previous observations. We further show that exciton-exciton repulsion and annihilation contribute nearly equally to non-linear propagation by disentangling the two processes in the experiment and simulations. Finally, we demonstrate effective shrinking of the light-emission over time across several 100's of picoseconds at the transition from exciton- to the plasma-dominated regimes. Supported by microscopic calculations for bandgap renormalization to identify Mott threshold, this indicates transient crossing between rapidly expanding, short-lived electron-hole plasma and slower, long-lived exciton populations.