Giant photogalvanic effect and second-harmonic generation in magnetic axion insulators*

Combining quantum perturbation theory and first-principles simulation, we predict giant nonlinear optical responses (NLOs) in even septuple layers of MnBi₂Te₄ family materials (MBTs), which are the zero-plateau quantum anomalous Hall systems (QAHs). The interlayer antiferromagnetic order breaks the inversion symmetry, and the amplitudes of injection current and second harmonic generation can be about one order of magnitude larger than those of the ferroelectrics, such as LiNbO₃ and BiFeO₃. Moreover, unlike the injection current in ferroelectrics, we find that the injection photocurrent only emerges under a linearly polarized light in MBTs. Our analysis indicates that these giant linearly-polarized second-order NLOs are resulted from the parity-time symmetry, three-fold rotation symmetry, and large spin-orbit coupling. These enhanced NLOs are valuable for characterizing subtle magnetic orders in QAHs and shed light on photo-detecting and photovoltaic applications based on emerging magnetic topological materials.