Symmetry-forbidden intervalley scattering by atomic defects in monolayer transition-metal dichalcogenides

Intervalley scattering by atomic defects in monolayer transition metaldichalcogenides (TDMs; MX₂) presents a serious obstacle for applicationsexploiting their unique valley-contrasting properties. Here, we show that thesymmetry of the atomic defects can give rise to an unconventional protectionmechanism against intervalley scattering in monolayer TMDs. The predicteddefectdependent selection rules for intervalley scattering can be verified viaFourier transform scanning tunneling spectroscopy (FT-STS), and provide aunique identification of, e.g., atomic vacancy defects (M vs X). Our findingsare consistent with the absence of the K K' intervalley FT-STS peak inrecent experiments.