Spin splitting in a MoS2 monolayer induced by exciton interaction

By pumping nonresonantly a ${\mathrm{MoS}}_{2}$ monolayer at 13 K under a circularly polarized continuous wave (CW) laser, we observe exciton energy redshifts that break the degeneracy between B excitons with opposite spin. The energy splitting increases monotonically with the laser power reaching as much as 18 meV, and it diminishes with the increase of temperature. The phenomenon can be explained theoretically by considering simultaneously the band-gap renormalization that gives rise to the redshift and exciton-exciton Coulomb exchange interaction that is responsible for the spin-dependent splitting. Our results offer a simple scheme to control the valley degree of freedom for ${\mathrm{MoS}}_{2}$ monolayers and provide an accessible method in investigating the many-body exciton-exciton interaction in such materials.