Determine the spin-conserving singlet-to-dark trion scattering time τ_SC in monolayer WS2

Determine the spin-conserving scattering time τ_SC that governs the conversion from the spin-singlet bright trion X_S^- to the dark trion X_D^- in monolayer WS2, where the process consists of the spin-conserving transfer of an electron from the upper conduction-band subband in the +K valley to the lower conduction-band subband in the −K valley and the conversion rate is defined as 1/τ_SC.

Background

In the paper’s analysis of valley dynamics and polarization, two key interconversion processes between trion species are highlighted: a spin-conserving transfer from the singlet bright trion (X_S^-) to the dark trion (X_D^-), and an electron–hole exchange-mediated transfer from the triplet bright trion (X_T^-) to the singlet trion (X_S^-). The authors cite an experimentally determined timescale for the exchange-mediated process (τ_EX ≈ 4 ps at 13 K), but emphasize that the characteristic timescale for the spin-conserving process has not yet been reported.

Establishing τ_SC is essential for quantitative modeling of valley depolarization pathways and for understanding the observed negative degree of circular polarization at the semi-dark trion energy. A precise value would inform rate-equation models and help resolve the hierarchy of scattering processes that control trion population imbalances across valleys in WS2.