Unconventional Magneto-Optical Effects in Altermagnets

TL;DR

This paper reveals that in ideal altermagnets, magneto-optical effects are primarily driven by the quantum metric rather than Berry curvature, expanding understanding of optical phenomena in these materials.

Contribution

It introduces a theoretical framework for unconventional MOEs in altermagnets dominated by quantum metric effects, supported by symmetry analysis and real material examples.

Findings

Unconventional MOEs dominate in ideal altermagnets.

Derived formulas quantify Berry curvature and quantum metric contributions.

Experimental predictions for MnTe and CrSBr are provided.

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.