Chirality of Valley Excitons in Monolayer Transition-Metal Dichalcogenides

TL;DR

This paper reveals that valley excitons in monolayer transition-metal dichalcogenides are chiral quasiparticles with finite orbital angular momentum, influencing their optical and magnetic properties, supported by ab-initio calculations matching experimental data.

Contribution

It introduces an ab-initio many-body theory describing valley excitons as chiral quasiparticles with orbital angular momentum in transition-metal dichalcogenides.

Findings

Valley excitons possess finite orbital angular momentum.

Excitons exhibit Zeeman splitting under magnetic fields.

Theoretical results agree with experimental measurements.

Abstract

By enabling control of valley degrees of freedom in transition-metal dichalcogenides, valley-selective circular dichroism has become a key concept in valleytronics. In this manuscript, we show that valley excitons -- bound electron-hole pairs formed at either the K or K' valleys upon absorption of circularly-polarized light -- are chiral quasiparticles characterized by a finite orbital angular momentum (OAM). We further formulate an ab-initio many-body theory of valley-selective circular dichroism and valley excitons based on the Bethe-Salpeter equation. Beside governing the interaction with circularly polarized light, the OAM confers excitons a finite magnetization which manifests itself through an excitonic Zeeman splitting upon interaction with external magnetic fields. The good agreement between our ab-initio calculations and recent experimental measurements of the exciton Zeeman shifts corroborate this picture.