Trion valley coherence in monolayer transition metal dichalcogenides*

Excitons and trions in monolayer transition metal dichalcogenides are formed at the boundary of the Brillouin zone at two inequivalent K and K’ valleys. Because of the valley contrasting optical selection rule, excitons and trions can be preferentially created using in a selected valley using light with a particular circular polariztion. Valley coherence — the coherent superposition between the excitonic states in the two valleys can be generated by a linearly polarized excitation. Exciton valley coherence has been detected as linearly polarized PL emission. However, the same method is not applicable to trions due to the spin-photon entanglement. With two-dimnensional Fourier transform spectroscopy, we explicitly measure the nonradiative exciton and trion valley coherence. Unlike exction, whose valley coherent time is limited by intrinsic fast population decay and electron-hole exchange interaction, our result shows that trion valley coherent time is limited by intra-valley pure dephasing processes.