Exciton valley depolarization in monolayer transition-metal dichalcogenides

Min Yang , Cédric Robert (1) , Zhengguang Lu , Dinh van Tuan , Dmitry Smirnov , Xavier Marie , Hanan Dery

1 Optoélectronique Quantique (LPCNO)

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 MoSe2 and MoTe2 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 WSe2, WS2 and likely MoS2 wherein the Rashba-induced coupling is negligible.

Domaines: Systèmes mésoscopiques et effet Hall quantique [cond-mat.mes-hall]

Min Yang, Cédric Robert, Zhengguang Lu, Dinh van Tuan, Dmitry Smirnov, et al.. Exciton valley depolarization in monolayer transition-metal dichalcogenides. Physical Review B, 2020, 101 (11), ⟨10.1103/PhysRevB.101.115307⟩. ⟨hal-03089585⟩