The discovery of two-dimensional van der Waals magnets has greatly expanded our ability to create and control nanoscale quantum phases. A unique capability emerges when a two-dimensional magnet is also a semiconductor that features tightly bound excitons with large oscillator strengths that fundamentally determine the optical response and are tunable with magnetic fields. 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, leading to a higher binding energy than excitons confined in bulk layers, and 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 unique confined excitons in a layered antiferromagnet, highlighting magnetic interactions as a vital approach for nanoscale quantum confinement, from few layers to the bulk limit.

Center for Computational Quantum Physics Flatiron Institute New York NY USA

Department of Applied Physics and Applied Mathematics Columbia University New York NY USA

Department of Chemistry Columbia University New York NY USA

Department of Inorganic Chemistry University of Chemistry and Technology Prague Prague Czech Republic

Department of Materials Science and Engineering University of Arizona Tucson AZ USA

Department of Mechanical Engineering Columbia University New York NY USA

Department of Physics and Regensburg Center for Ultrafast Nanoscopy University of Regensburg Regensburg Germany

Department of Physics Columbia University New York NY USA

Department of Physics Pennsylvania State University University Park PA USA

Department of Physics Technical University of Munich Munich Germany

Institute for Molecules and Materials Radboud University Nijmegen Netherlands

Institute of Applied Physics and Würzburg Dresden Cluster of Excellence ct qmat TUD Dresden University of Technology Dresden Germany

Munich Center for Quantum Science and Technology Technical University of Munich Garching Germany

National Renewable Energy Laboratory Golden CO USA

Theory and Simulation of Condensed Matter King's College London London UK

Zentrum für Quantum Engineering Technical University of Munich Garching Germany

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Magnon-mediated exciton-exciton interaction in a van der Waals antiferromagnet