Fig. 1

Schematic illustration of the band structure and optical transitions of monolayer MoX2 and WX2. Shown are the spin-dependent band structures near the K and K′ points, derived from first-principles calculations12, including the (helicity-selective) optical transitions and electronic configurations involved in A excitons and associated negatively charged trions (the lower spin–split valence band is not shown). Singlet trions are intervalley in MoX2 but intravalley in WX2; the situation is reversed for triplet trions. Note that the depicted vertical transitions become mixed as a result of quasiparticle interactions