Exciton valley depolarization in monolayer transition-metal dichalcogenides

Abstract

The valley degree of freedom is a sought-after quantum number in monolayer transition-metal dichalcogenides. Similar to optical spin orientation in semiconductors, the helicity of absorbed photons can be relayed to the valley (pseudospin) quantum number of photoexcited electrons and holes. Also similar to the quantum-mechanical spin, the valley quantum number is not a conserved quantity. Valley depolarization of excitons in monolayer transition-metal dichalcogenides due to long-range electron-hole exchange typically takes a few ps at low temperatures. Exceptions to this behavior are monolayers MoSe${}_2$ and MoTe${}_2$ wherein the depolarization is much faster. We elucidate the enigmatic anomaly of these materials, finding that it originates from Rashba-induced coupling of the dark and bright exciton branches next to their degeneracy point. When photoexcited excitons scatter during their energy relaxation between states next to the degeneracy region, they reach the light cone after losing the initial helicity. The valley depolarization is not as fast in monolayers WSe${}_2$, WS${}_2$ and likely MoS${}_2$ wherein the Rashba-induced coupling is negligible.

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Cited by in corpus (19)

• Measurement of the Spin-Forbidden Dark Excitons in MoS2 and MoSe2 monolayers
• Relaxation and darkening of excitonic complexes in electrostatically-doped monolayer semiconductors: Roles of exciton-electron and trion-electron interactions
• Optical read-out of Coulomb staircases in a moiré superlattice via trapped interlayer trions
• Prolonging Valley Polarization Lifetime through Gate-Controlled Exciton-to-Trion Conversion in Monolayer Molybdenum Ditelluride
• Enhancement of Exciton Valley Polarization in Monolayer MoS2 Induced by Scattering
• Strain control of exciton and trion spin-valley dynamics in monolayer transition metal dichalcogenides
• Exciton fine structure splitting and linearly polarized emission in strained transition-metal dichalcogenide monolayers
• Control of the exciton valley dynamics in van der Waals heterostructures
• Excitons and trions in monolayer semiconductors with correlated electrons
• Intervalley electron-hole exchange interaction and impurity-assisted recombination of indirect excitons in WS${}_2$ and WSe${}_2$ monolayers
• Ultrafast Chirality-dependent Dynamics from Helicity-resolved Transient Absorption Spectroscopy
• Photonics in Flatland: Challenges and Opportunities for Nanophotonics with 2D Semiconductors
• Valley-mediated singlet- and triplet-polaron interactions and quantum dynamics in a doped WSe${}_2$ monolayer
• Coherent spin dynamics of excitons in strained monolayer semiconductors
• Valley-exchange coupling probed by angle-resolved photoluminescence
• Energy shifts and broadening of excitonic resonances in electrostatically-doped semiconductors
• Valley polarization fluctuations, bistability, and switching in two-dimensional semiconductors
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• Excitonic Transport and Intervalley Scattering in Exfoliated MoSe2 Monolayer Revealed by Four-Wave-Mixing Transient Grating Spectroscopy