Unconventional Magneto-Optical Effects in Altermagnets

Abstract

The ideal altermagnets are a class of collinear, crystal-symmetry-enforced fully compensated magnets with nonrelativistic spin-split bands, in which contributions from Berry curvature to magneto-optical effects (MOEs) are strictly forbidden by an effective time-reversal symmetry. Here we show that, in such systems, MOEs are exclusively induced by the quantum metric and, in realistic altermagnets, are typically dominated by it. We refer to Berry-curvature-induced MOEs as conventional MOEs and to quantum-metric-dominated MOEs as unconventional MOEs. We derive general formulas that incorporate both Berry curvature and quantum metric for unconventional MOEs in altermagnets, enabling a quantitative evaluation of their respective contributions. Through symmetry analysis, we prove that ideal altermagnets are constrained to exhibit only unconventional MOEs. Using the three-dimensional canonical altermagnet MnTe and the emerging two-dimensional bilayer twisted altermagnet CrSBr as illustrative examples, we demonstrate that unconventional MOEs are prevalent in altermagnets. Our results establish altermagnets as a natural platform for quantum-metric-driven optical phenomena, substantially broadening the scope of MOEs and providing concrete predictions that can be tested in future experimental studies.

Figures (2)

• Figure 1: The unconventional magneto-optical effects in 3D altermagnet MnTe. (a) The magnetic unit cell of MnTe, as drawn using VESTA vesta. The polarization direction of the incident light lies within the $ac$ plane, forming an angle of 5° with respect to the $a$-axis. (b) The DFT spin splitting band structure without SOC. The positions of the four momenta are given by $\mathrm{K}_1=(1/3,1/3,1/4)$, $\mathrm{K}_2=(1/3,-2/3,1/4)$, $\mathrm{K}_3=(1/3,1/3,-1/4)$, $\mathrm{K}_4=(1/3,-2/3,-1/4)$. (c) The DFT band structure with SOC. (d) The longitudinal optical conductivities $\sigma_{xx}$ and $\sigma_{zz}$. (e) The transverse conductivities $\sigma_{zx}\equiv\sigma_{zx}^g+\sigma_{zx}^\Omega$. $\sigma_{zx}$ is divided into the symmetric part $\sigma_{zx}^g$ and the antisymmetric part $\sigma_{zx}^{\Omega}$. (f) The calculated Kerr and specific Faraday angles when the Berry curvature exclusively contributes to the transverse optical conductivities. (g) The calculated Kerr and specific Faraday angles when the contribution comes exclusively from the quantum metric. (h) The Kerr angles and specific Faraday angles with the consideration of both the Berry curvature and quantum metric. The calculations in (d)--(h) have all incorporated SOC.
• Figure 2: The unconventional magneto-optical effects in 2D bilayer twisted altermagnet CrSBr. (a) Top view of the unit cell of the twisted bilayer CrSBr with a twist angle 73.44°. The polarization direction of the incident light lies within the $ab$ plane, forming an angle of 45° with respect to the $a$-axis. (b) The magnetic unit cell of twisted bilayer CrSBr. (c) The DFT spin splitting band structure without SOC. (d) The DFT band structure with SOC. (e) The longitude optical conductivities $\sigma_{xx}$ and $\sigma_{yy}$. (f) The transverse conductivities $\sigma_{xy}\equiv\sigma_{xy}^g+\sigma_{xy}^\Omega$. $\sigma_{xy}$ is divided into symmetric part $\sigma_{xy}^g$ and antisymmetric part $\sigma_{xy}^{\Omega}$. (g) The Kerr angles and Faraday angles. Given that the Berry curvature does not contribute to the MOEs, we only present the total Kerr and Faraday angles. The calculations in (e)--(g) have all incorporated SOC.