Spin and valley dynamics of excitons in transition metal dichalcogenides monolayers

TL;DR

This paper reviews the spin and valley dynamics of excitons in monolayer transition metal dichalcogenides, highlighting the role of long-range exchange interactions in exciton mixing and recent experimental insights.

Contribution

It provides a comprehensive overview of the exciton fine structure, physical origin of exchange interactions, and models spin dynamics in monolayer TMDs, connecting theory with recent experiments.

Findings

Long-range exchange interaction causes rapid bright exciton mixing.

Model explains spin dynamics observed in time-resolved photoluminescence.

Theoretical approaches align with Kerr rotation experimental data.

Abstract

Monolayers of transition metal dichalcogenides, namely, molybdenum and tungsten disulfides and diselenides demonstrate unusual optical properties related to the spin-valley locking effect. Particularly, excitation of monolayers by circularly polarized light selectively creates electron-hole pairs or excitons in non-equivalent valleys in momentum space, depending on the light helicity. This allows studying the inter-valley dynamics of charge carriers and Coulomb complexes by means of optical spectroscopy. Here we present a concise review of the neutral exciton fine structure and its spin and valley dynamics in monolayers of transition metal dichalcogenides. It is demonstrated that the long-range exchange interaction between an electron and a hole in the exciton is an efficient mechanism for rapid mixing between bright excitons made of electron-hole pairs in different valleys. We discuss the physical origin of the long-range exchange interaction and outline its derivation in both the electrodynamical and $\mathbf k \cdot \mathbf p$ approaches. We further present a model of bright exciton spin dynamics driven by an interplay between the long-range exchange interaction and scattering. Finally, we discuss the application of the model to describe recent experimental data obtained by time-resolved photoluminescence and Kerr rotation techniques.