  Although single-layer transition-metal dichalcogenides with novel valley
functionalities are promising candidate to realize valleytronic devices, the
essential understanding of valley depolarization mechanisms is still
incomplete. Based on pump-probe experiments performed for MoSe2 and WSe2
monolayers and corroborating analysis from density functional calculations, we
demonstrate that coherent phonons at the K-point of the Brillouin zone can
effectively mediate the valley transfer of electron carriers. In the MoSe2
monolayer case, we identify this mode as the flexural acoustic ZA(K) mode,
which has broken inversion symmetry and thus can enable electron spin-flip
during valley transfer. On the other hand, in the monolayer WSe2 case where
spin-preserving inter-valley relaxations are preferred coherent LA(K) phonons
with even inversion symmetry are efficiently generated. These findings
establish that, while the specifics of inter-valley relaxations depend on the
spin alignments of energy bands, the K-point phonons should be taken into
account as an effective valley depolarization pathway in transition metal
dichalcogenide monolayers.
