  Intervalley scattering by atomic defects in monolayer transition metal
dichalcogenides (TDMs; MX2) presents a serious obstacle for applications
exploiting their unique valley-contrasting properties. Here, we show that the
symmetry of the atomic defects can give rise to an unconventional protection
mechanism against intervalley scattering in monolayer TMDs. The predicted
defect-dependent selection rules for intervalley scattering can be verified via
Fourier transform scanning tunneling spectroscopy (FT-STS), and provide a
unique identification of, e.g., atomic vacancy defects (M vs X). Our findings
put the absence of the intervalley FT-STS peak in recent experiments in a
different perspective.
