Spin and valley dynamics of excitons in transition metal dichalcogenide monolayers

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 k·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. Kerr rotation dynamics at T=4K for a σ⁺ and σ^- pump beam in WSe₂. Inset: Schematics of the optical selection rules of the excitons photogenerated from charge carriers in K_± valleys and their coupling induced by the long-range exchange interaction.