Giant Magneto-Optical Effects in Two-Dimensional Flat-Band Antiferromagnets.

In this work, we reveal giant magneto-optical responses in two-dimensional (2D) antiferromagnets with nearly flat electronic bands, based on first-principles calculations and group-theoretical analysis. We identify a record-large second-order magneto-optical Schäfer-Hubert (SH) effect-featuring a polarization rotation angle of 28 ∘ -in monolayer antiferromagnetic RuOCl 2 , driven by flat-band-enhanced interband optical transitions. Both the valence and conduction bands exhibit pronounced directional flatness, giving rise to highly anisotropic optical absorption and broadband hyperbolic frequency windows spanning the entire visible spectrum. This anisotropy leads to an exceptionally strong linear dichroism (LD) reaching 50%, far exceeding values reported in other 2D magnetic systems. Remarkably, the giant SH effect and LD appear at distinct photon energies, reflecting a momentum-direction-dependent crossover between flat and dispersive bands. Both responses are further amplified with increasing RuOCl 2 film thickness. Our results establish flat-band antiferromagnets as a fertile platform for realizing giant nonlinear magneto-optical effects and open new avenues for 2D opto-spintronic device applications.