Bulk and Surface Excitons in the van der Waals Magnet CrSBr: Magneto-Optical Studies to 55 T

In thin layers of the 2D magnetic semiconductor CrSBr, very recent studies identified two distinct band-edge optical resonances, believed to arise from distinguishable bulk and surface excitons. This behavior reportedly originates from the highly anisotropic nature of CrSBr─particularly in its antiferromagnetic state─where excitons are effectively confined within individual monolayers, such that excitons in the two surface layers "see" a different local dielectric environment and have a lower resonance energy. To explore this scenario, here we investigate optical absorption properties of few-layer CrSBr in magnetic fields. In addition to the fundamental exciton resonance at ∼1.36 eV, we observe an absorption resonance ∼20 meV lower in energy. Compared to the fundamental transition, this resonance redshifts only half as much in small magnetic fields that induce ferromagnetic order, while in high fields to 55 T it exhibits a smaller diamagnetic shift. Both behaviors point to distinguishable populations of bulk and surface excitons in CrSBr.