Microscopic origin of anomalous interlayer exciton transport in van der Waals heterostructures

TL;DR

This paper develops a microscopic theory for interlayer exciton interactions in van der Waals heterostructures, revealing how their transport can be tuned by environmental factors and explaining counter-intuitive propagation behaviors.

Contribution

It introduces a detailed microscopic model accounting for exciton dipolar and fermionic properties, explaining non-linear transport phenomena in TMD heterostructures.

Findings

Interlayer exciton propagation is highly non-linear at elevated densities.

Environmental screening significantly influences exciton transport speed.

Counter-intuitively, free-standing samples show slower exciton propagation than encapsulated ones.

Abstract

Van der Waals heterostructures constitute a platform for investigating intriguing many-body quantum phenomena. In particular, transition-metal dichalcogenide (TMD) hetero-bilayers host long-lived interlayer excitons which exhibit permanent out-of-plane dipole moments. Here, we develop a microscopic theory for interlayer exciton-exciton interactions including both the dipolar nature of interlayer excitons as well as their fermionic substructure, which gives rise to an attractive fermionic exchange. We find that these interactions contribute to a drift force resulting in highly non-linear exciton propagation at elevated densities in the MoSe$_2$-WSe$_2$ heterostructure. We show that the propagation can be tuned by changing the number of hBN spacers between the TMD layers or by adjusting the dielectric environment. In particular, although counter-intuitive, we reveal that interlayer excitons in free-standing samples propagate slower than excitons in hBN-encapsulated TMDs - due to an enhancement of the net Coulomb-drift with stronger environmental screening. Overall, our work contributes to a better microscopic understanding of the interlayer exciton transport in technologically promising atomically thin semiconductors.