Magnetically confined surface and bulk excitons in a layered antiferromagnet

The discovery of two-dimensional (2D) van der Waals magnets has greatly expanded our ability to create and control nanoscale quantum phases. A unique capability emerges when a 2D magnet is also a semiconductor, featuring tightly bound excitons with large oscillator strengths that fundamentally determining the optical response and are tunable with magnetic fields. Structural and magnetic anisotropies can lead to anisotropic 2D excitons and even hybrid-dimensional excitons. Here we report a previously unidentified type of optical excitation – a magnetic surface exciton – enabled by the antiferromagnetic spin correlations that confine excitons to the surface of CrSBr. Magnetic surface excitons exhibit stronger Coulomb attraction and thus a higher binding energy than excitons confined in bulk layers, which profoundly alter the optical response of few-layer crystals. Distinct magnetic confinement of surface and bulk excitons is established by layer- and temperature-dependent exciton reflection spectroscopy and corroborated by ab initio many-body perturbation theory calculations. By quenching interlayer excitonic interactions, the antiferromagnetic order of CrSBr strictly confines the bound electron-hole pairs within the same layer, regardless of the total number of layers. Our work unveils novel confined excitons in a layered antiferromagnet, highlighting magnetic interactions as a vital approach for nanoscale quantum confinement, from few layers to the bulk limit.