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Time-resolved spectroscopy of coupled spin-valley-dynamics in monolayer transition metal dichalcogenides at low temperatures

We demonstrate the exfoliation of large-area monolayer flakes prepared from bulk MoS2 crystals. The flakes are first characterized using Raman and photoluminescence measurements. We then utilize time-resolved Kerr rotation (TRKR) measurements to probe the valley dynamics in the monolayer flakes at low temperatures. This technique allows resonant excitation of the excitonic transitions and yields sub-picosecond resolution. We find valley lifetimes of about 40 ps at a temperature of 4 K in monolayer MoS2 for resonant excitation. With increasing temperatures, we observe a dramatic decrease of the valley lifetimes, indicating that valley dephasing is mediated by phonon-related scattering processes.