Temperature Dependent Valley Relaxation Dynamics in Single Layer WS2 Measured Using Ultrafast Spectroscopy

Abstract

We measured the lifetime of optically created valley polarization in single layer WS2 using transient absorption spectroscopy. The electron valley relaxation is very short (< 1ps). However the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single layer WS2. One process involves direct scattering of excitons from K to K' valleys with a spin flip-flop interaction. The other mechanism involves scattering through spin degenerate Gamma valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between Gamma and K valleys.

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• Two Dimensional Atomic Crystals
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• Dark excitons and the elusive valley polarization in transition metal dichalcogenides
• Understanding Variations in Circularly Polarized Photoluminescence in Monolayer Transition Metal Dichalcogenides
• Observation of intervalley biexcitonic optical Stark effect in monolayer WS2
• Symmetry and Control of Spin-Scattering Processes in Two-Dimensional Transition Metal Dichalcogenides
• Valley Dynamics of Excitons in Monolayer Dichalcogenides
• Theory of the Coherent Response of Magneto-Excitons and Magneto-Biexcitons in Monolayer Transition Metal Dichalcogenides
• Symmetry-breaking and spin-blockage effects on carrier dynamics in single-layer tungsten diselenide
• Excitonic Transport and Intervalley Scattering in Exfoliated MoSe2 Monolayer Revealed by Four-Wave-Mixing Transient Grating Spectroscopy