K-point longitudinal acoustic phonons are responsible for ultrafast intervalley scattering in monolayer MoSe.

In transition metal dichalcogenides, valley depolarization through intervalley carrier scattering by zone-edge phonons is often unavoidable. Although valley depolarization processes related to various acoustic phonons have been suggested, their optical verification is still vague due to nearly degenerate phonon frequencies on acoustic phonon branches at zone-edge momentums. Here we report an unambiguous phonon momentum determination of the longitudinal acoustic (LA) phonons at the K point, which are responsible for the ultrafast valley depolarization in monolayer MoSe. Using sub-10-fs-resolution pump-probe spectroscopy, we observed coherent phonons signals at both even and odd-orders of zone-edge LA mode involved in intervalley carrier scattering process. Our phonon-symmetry analysis and first-principles calculations reveal that only the LA phonon at the K point, as opposed to the M point, can produce experimental odd-order LA phonon signals from its nonlinear optical modulation. This work will provide momentum-resolved descriptions of phonon-carrier intervalley scattering processes in valleytronic materials.