Large Exciton Binding Energy in the Bulk Van Der Waals Magnet CrSBr

Excitons, bound electron-hole pairs, influence the optical properties instrongly interacting solid state systems. Excitons and their associatedmany-body physics are typically most stable and pronounced in monolayermaterials. Bulk systems with large exciton binding energies, on the other hand,are rare and the mechanisms driving their stability are still relativelyunexplored. Here, we report an exceptionally large exciton binding energy insingle crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizingstate-of-the-art angle-resolved photoemission spectroscopy and self-consistentab-initio GW calculations, we present direct spectroscopic evidence that robustelectronic and structural anisotropy can significantly amplify the excitonbinding energy within bulk crystals. Furthermore, the application of a verticalelectric field enables broad tunability of the optical and electronicproperties. Our results indicate that CrSBr is a promising material for thestudy of the role of anisotropy in strongly interacting bulk systems and forthe development of exciton-based optoelectronics.