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Inversion symmetry breaking and three-fold rotation symmetry grant the valleydegree of freedom to the robust exciton in monolayer transition metaldichalcogenides (TMDCs), which can be exploited for valleytronics applications.However, the short lifetime of the exciton significantly constrains thepossible applications. In contrast, dark exciton could be long-lived but doesnot necessarily possess the valley degree of freedom. In this work, we reportthe identification of the momentum-dark, intervalley exciton in monolayer WSe2through low-temperature magneto-photoluminescence (PL) spectra. Interestingly,the intervalley exciton is brightened through the emission of a chiral phononat the corners of the Brillouin zone (K point), and the pseudoangular momentum(PAM) of the phonon is transferred to the emitted photon to preserve the valleyinformation. The chiral phonon energy is determined to be ~ 23 meV, based onthe experimentally extracted exchange interaction (~ 7 meV), in excellentagreement with the theoretical expectation of 24.6 meV. The long-livedintervalley exciton with valley degree of freedom adds an excitingquasiparticle for valleytronics, and the coupling between the chiral phonon andintervalley exciton furnishes a venue for valley spin manipulation.

Momentum-Dark Intervalley Exciton in Monolayer Tungsten Diselenide Brightened via Chiral Phonon