Anomalous enhancement of valley polarization in multilayer WS₂ at room temperature

We probe the polarization of the "A" exciton photoluminescence from monolayer and multilayer WS₂ at 10 K and 295 K under near-resonant and off-resonant conditions. The monolayer WS₂ exhibits relatively low valley polarization, around 24% at 10 K and 8% at 295 K, while all multilayer WS₂ samples show very high valley polarization, which is a more or less constant value of around 80% at 10 K under near-resonant excitation. At room temperature, it is observed experimentally that valley polarization in multilayer WS₂ monotonously increases with shrinking of the indirect bandgap energy. The phonon-assisted intervalley scattering via the K-Γ-K(K') valleys is identified as the primary valley polarization relaxation channel, which could be gradually suppressed as the thickness increases, leading to a valley polarization of up to 70% in multilayer WS₂ (>3 unit layers).