Exciton hybridization and optical anisotropy in twin-stacked magnetic CrSBr bilayer

Two-dimensional van der Waals magnetic semiconductor CrSBr is of emerging interest for its combination of anisotropic electronic and magnetic properties, its stability under ambient conditions, and its potential applications in spintronics. CrSBr bilayers exhibit intralayer ferromagnetism and interlayer antiferromagnetism, the latter suppressing interlayer hybridization and localizing electronic states in one or the other layer. Unlike other prominent van der Waals materials (such as graphene and transition metal dichalcogenides), CrSBr possesses a 2D rectangular lattice, and a recent experiment [1] has reported so-called twin-twisted bilayers, where strong exciton-magnon coupling between twin-oriented layers has been observed. Here, we use ab initio density functional theory and GW-Bethe-Salpeter equation calculations to study the electronic structure of twin-stacked and other twist-angle structures. Our calculations demonstrate that the strength of the interlayer hybridization depends on the relative angle between spins in the two layers and fine details of the atomic structure. Moreover, we find that the two lowest bright excitons, which are degenerate and isolated in opposite individual layers at zero twist angle, hybridize with twist angle, resulting in a highly-tunable optical polarization anisotropy and noncollinear interlayer magnetic configurations, consistent with experiments. Our work reveals the detailed nature and coupling of the electronic structure, magnetic, and optical properties in novel twin-stacked CrSBr bilayers.