  Excitons, bound electron-hole pairs, influence the optical properties in
strongly interacting solid state systems. Excitons and their associated
many-body physics are typically most stable and pronounced in monolayer
materials. Bulk systems with large exciton binding energies, on the other hand,
are rare and the mechanisms driving their stability are still relatively
unexplored. Here, we report an exceptionally large exciton binding energy in
single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing
state-of-the-art angle-resolved photoemission spectroscopy and self-consistent
ab-initio GW calculations, we present direct spectroscopic evidence that robust
electronic and structural anisotropy can significantly amplify the exciton
binding energy within bulk crystals. Furthermore, the application of a vertical
electric field enables broad tunability of the optical and electronic
properties. Our results indicate that CrSBr is a promising material for the
study of the role of anisotropy in strongly interacting bulk systems and for
the development of exciton-based optoelectronics.
