Observation of anomalous Hall resonance of massive Dirac fermions in topological kagome-lattice magnet

The kagome-lattice materials promise emergence of Dirac fermions thanks to the special lattice geometry, which potentially realizes intriguing quantum topological states through various many-body interactions. The low-energy electromagnetic phenomena arising from such the Dirac fermions are expected to show the remarkable enhancement and, in certain conditions, to approach the universal responses, which, however, have remained elusive experimentally. Here, we show the resonantly enhanced magneto-optical response of massive Dirac fermions in kagome-lattice magnet TbMn6Sn6. The infrared magneto-optical spectroscopy reveals that the interband transition on massive Dirac bands significantly contributes to the observed resonance in the optical Hall conductivity. The analytical model expressed by a few band parameters reproduces the spectral characteristics of the resonance, which robustly produces almost 20 % of the quantized Hall conductance per one kagome layer even at room temperature. Our findings establish the general optical response of massive Dirac fermions, which is closely related to the universal electrodynamics in quantum anomalous Hall state.