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 ${\mathrm{MoSe}}_2$ and ${\mathrm{MoTe}}_2$ 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 ${\mathrm{WSe}}_2$, ${\mathrm{WS}}_2$, and ${\mathrm{MoS}}_2$, wherein the degeneracy is absent resulting in negligible Rashba-induced coupling between bright and dark excitons.

中文翻译：

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单层过渡金属二卤化物中的激子谷去极化

谷的自由度是单层过渡金属二卤化碳中的抢手量子数。类似于半导体中的光学自旋取向，吸收的光子的螺旋性可以传递给光激发电子和空穴的谷（伪自旋）量子数。同样类似于量子力学自旋，谷量量子数不是守恒量。由于长距离电子-空穴交换，单层过渡金属二硫属化合物中激子的谷型去极化通常在低温下花费几ps。这种行为的例外是单层${\mathrm{钼硒}}_2$ 和 ${\mathrm{Te}}_2$其中去极化要快得多。我们阐明了这些材料的神秘异常，发现它起源于Rashba诱导的简并点附近暗暗激子分支的耦合。当光激发的激子在能量退化期间在简并区域附近的状态之间散射时，它们失去初始螺旋度后到达光锥。单层中的谷底去极化速度不那么快${\mathrm{硒化钨}}_2$， ${\mathrm{WS}}_2$和 ${\mathrm{硫化钼}}_2$，其中不存在简并性，导致在明亮和黑暗激子之间由Rashba诱导的偶联可忽略不计。