Probing the quantum geometry and topology in condensed matter systems has relied heavily on static electronic transport experiments in magnetic fields. Yet, contact-free optical measurements have rarely been explored. Here, we report the observation of resonant magnetic circular dichroism (MCD) in the infrared range in thin film MnBi_{2}Te_{4} exhibiting a spectral intensity that correlates with the anomalous Hall effect. Both phenomena emerge with a field-driven phase transition from an antiferromagnet to a canted ferromagnet. By theoretically relating the MCD to the anomalous Hall effect via Berry curvature for a metallic state, we show that this transition accompanies an abrupt onset of Berry curvature, signaling a topological phase transition from a topological insulator to a doped Chern insulator. Our density functional theory calculation suggests the MCD signal mainly originates from an optical transition at the Brillouin zone edge, hinting at a potential new source of Berry curvature away from the commonly considered Γ point. Our findings demonstrate a novel experimental approach for detecting Berry curvature through spectroscopy of the interband MCD, generally applicable to magnetic materials.

Charles University Institute of Physics Ke Karlovu 5 Prague 121 16 Czech Republic

Institut des NanoSciences de Paris UPMC CNRS 4 place Jussieu 75005 Paris France

Johannes Kepler Universität Institut für Halbleiter und Festkörperphysik 4040 Linz Austria

KU Leuven Quantum Solid State Physics Leuven Belgium

National High Magnetic Fields Laboratory Florida State University Tallahassee Florida 32310 USA

Univ Toulouse Univ Grenoble Alpes LNCMI EMFL CNRS UPR3228 Univ Toulouse 3 INSA T Grenoble and Toulouse France

University of Notre Dame Department of Physics and Astronomy Notre Dame Indiana 46556 USA

University of Notre Dame Notre Dame Integrated Imaging Facility Notre Dame Indiana 46556 USA

Virginia Tech Department of Physics Blacksburg Virginia 24061 USA

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