Fig. 3: Exciton spin relaxation dominated by the electron–hole exchange interaction.

a The σ+ σ+ and σ+ σ− TA kinetics probed at ~2.35 eV, extracted from Fig. 1b, c, respectively. The gray curve shows the absolute value of the σ+ σ− TA kinetics. b The schematics of possible spin depolarization mechanisms for photo-injected +1 excitons. The W_e,h represents the spin flip of electron or hole, and W_ex is the electron-hole exchange interaction. K and K₀ are the momentum of excitons and light, respectively. The horizontal dotted lines indicate the originally degenerate spin states. Following photoexcitation with a σ+ laser pulse, +1 exciton states shows a large blueshift (blue line) and −1 exciton states exhibits a slight redshift (red line). c Normalized kinetics of the circular dichroism signal CD_ΔT/T, which is defined as ΔT/T_{σ+ σ+,Δt}−ΔT/T_{σ+ σ−,Δt} and averaged in the probed energy range of 2.32–2.38 eV. The colored symbols are experimental results at different pump fluence, fitted with bi-exponential decay curves (black lines). d The spin decay times extracted from c as a function of pump fluence. The τ_Wex is the exciton spin decay times due to e–h exchange interactions, while τ_f is the initial fast-decay component as discussed in the text. The dashed lines are guide for the eye.