Monolayer transition-metal dichalcogenides, such as WSe₂, are direct gap, multi-valley semiconductors. Long-range electron-hole exchange interactions mix the valleys, yielding dispersion relations for massive (Q²) as well as light-like (Q) excitons. We report magneto-photoluminescence spectroscopy of excitons in the monolayer semiconductor WSe₂ to B = 25T. The magnetic field-dependent line shape of the neutral exciton reveals the emergence of a new blue-detuned emission peak in both field orientations. Analyzing the distinct magnetic field-dependent shifts of both peaks facilitates the identification of the emergent feature as a spin-singlet with a significantly smaller reduced exciton mass as compared to the neutral exciton. The intensity of the emergent feature increases with magnetic field according to B², as expected for a linear dispersion relation. The density-dependent diamagnetic shift ratios of both features follow the expected density dependence of the electron-hole exchange interactions. We interpret our observations within a picture of magnetic-field-induced coupling between the bright massive and quasi dark light-like exciton, leading to its brightening.
Delhomme et al. (Tue,) studied this question.