Berry curvature induced magnetic circular dichroism in magnetic topological insulators and beyond.

MnBi2Te4 is an established magnetic topological insulator exhibiting Chern insulator physics in two-dimensions when the Fermi level is tuned to the bulk gap. If the Fermi level crosses the conduction or valence band, a non-quantized anomalous Hall effect is still measured [1], but the origin of this anomalous Hall conductivity is debated. We show that a correspondence between the anomalous Hall effect and the magnetooptical response of the material manifesting as a resonant magnetic circular dichroism (MCD) at interband energies [2] evidences the changing intrinsic Berry curvature of this material as a function of magnetic field. In MBE grown thin films of MnBi2Te4, we experimentally measure an MCD signal in the mid-infrared exhibiting a spectral intensity that correlates with the field dependence of the anomalous Hall effect [3]. Both phenomena emerge with a field-driven magnetic phase transition from an antiferromagnet to a canted ferromagnet. This transition accompanies an abrupt onset of Berry curvature, signaling a topological phase transition from a topological insulator to a doped Chern insulator. The interband nature of the MCD response ties the anomalous Hall conductivity to a Berry curvature band origin, removing the need to carry out a scaling analysis. Our findings demonstrate a contact-free experimental approach for detecting Berry curvature through spectroscopy of the interband MCD, generally applicable to magnetic topological materials and potentially relevant for some altermagnets.

[1] Bac et al., npj Quantum Materials 7 46 (2022), Lee et al., Phys. Rev. Research 1, 012011 (2019).

[2] Souza and Vanderbilt, Phys. Rev. B Phys. Rev. B 77, 054438 (2008).

[3] Bac, le Mardele et al., Phys. Rev. Lett. 134 016601 (2025).