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

TL;DR

This paper identifies a long-lived, momentum-dark intervalley exciton in monolayer WSe2 that is brightened via chiral phonon emission, enabling potential valleytronics applications through phonon-exciton coupling.

Contribution

It reports the first observation of a brightened momentum-dark intervalley exciton in monolayer WSe2 mediated by chiral phonons, revealing new pathways for valley spin manipulation.

Findings

Intervalley exciton is brightened by chiral phonon emission.

Chiral phonon energy measured at ~23 meV, matching theoretical predictions.

Long-lived exciton with valley degree of freedom demonstrated.

Abstract

Inversion symmetry breaking and three-fold rotation symmetry grant the valley degree of freedom to the robust exciton in monolayer transition metal dichalcogenides (TMDCs), which can be exploited for valleytronics applications. However, the short lifetime of the exciton significantly constrains the possible applications. In contrast, dark exciton could be long-lived but does not necessarily possess the valley degree of freedom. In this work, we report the identification of the momentum-dark, intervalley exciton in monolayer WSe2 through low-temperature magneto-photoluminescence (PL) spectra. Interestingly, the intervalley exciton is brightened through the emission of a chiral phonon at 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 valley information. The chiral phonon energy is determined to be ~ 23 meV, based on the experimentally extracted exchange interaction (~ 7 meV), in excellent agreement with the theoretical expectation of 24.6 meV. The long-lived intervalley exciton with valley degree of freedom adds an exciting quasiparticle for valleytronics, and the coupling between the chiral phonon and intervalley exciton furnishes a venue for valley spin manipulation.