Temperature- and layer- dependent spin-orbit coupling in atom-thick transition metal dichalcogenides*

Atomically thin semiconducting transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring two-dimensional exciton physics. These excitons play a crucial role in determining the light-matter interactions of a semiconducting material. Understanding the origin of valence band splitting is important because it governs the unique spin and valley physics in TMDs. We have explored the effects of temperature and number of layers on spin−orbit coupling (SOC) in MoS₂ and WS₂ (monolayers to 6 Layers and bulk~100 layers) by determining the difference between A and B excitonic peak energy (Δ=E_B-E_A) in photocurrent spectra at different temperatures from 77 K to 300 K. We have found few important characteristics of the SOC coupling. First, MoS₂ and WS₂ demonstrate very different temperature-dependent SOC behavior. Second, SOC strength is the lowest for monolayer. Third, coupling increases as we increase the layer thickness. Third, the rate of change of the coupling strength with respect to temperature (m=δΔ/δT) depends on the layer thickness for MoS₂. Our study sheds light on the distinctive behaviors about SOC coupling in layered MoS₂ and WS₂.