Exciton dispersion across the spin-flip transition in CrSBr

CrSBr is a layered magnetic semiconductor that remains magnetically ordered and air stable down to a monolayer. CrSBr exhibits an exciton with large oscillator strength that is strongly coupled to the magnetism, making this material an ideal platform for functional devices integrating optoelectronic and spintronic capabilities. However, many questions remain about this spin-exciton coupling, optically forbidden “dark” excitons, and finite momentum properties. We have carried out resonant inelastic x-ray scattering (RIXS) at the Cr L3 edge on CrSBr to probe the full spectrum of bright and dark excitons and their momentum dependence. By applying a magnetic field, we tracked the RIXS spectrum across the spin-flip transition.

We find a rich spectrum that includes the bright excitons seen in optical measurements, as well as several dark excitons. The momentum dependence shows an anisotropic dispersion that resembles the single particle conduction bands. Across the spin-flip transition, the lowest energy bright and dark excitons are uniformly redshifted by 15 meV along the Γ-X direction, while showing more complex behavior along the Γ-Y direction. These results yield insight into the electronic character and magnetic coupling of the unconventional excitons in CrSBr.