  Monolayer transition metal dichalcogenide (TMDC) crystals, as direct-gap
materials with unusually strong light-matter interaction, have attracted much
recent attention. In contrast to the initial understanding, the minima of the
conduction band are predicted to be spin split. Because of this splitting and
the spin-polarized character of the valence bands, the lowest-lying excitonic
states in WX2 (X=S, Se) are expected to be spin-forbidden and optically dark.
To date, however, there has been no direct experimental probe of these dark
band-edge excitons, which strongly influence the light emission properties of
the material. Here we show how an in-plane magnetic field can brighten the dark
excitonic states and allow their properties to be revealed experimentally in
monolayer WSe2. In particular, precise energy levels for both the neutral and
charged dark excitons were obtained and compared with ab-initio calculations
using the GW-BSE approach. Greatly increased emission and valley lifetimes were
observed for the brightened dark states as a result of their spin
configuration. These studies directly probe the excitonic spin manifold and
provide a new route to tune the optical and valley properties of these
prototypical two-dimensional semiconductors.
