Unveiling Hidden Magnons with Anomalous Rotational Symmetry

TL;DR

This study reveals how Mn doping in Ca$_2$RuO$_4$ uncovers hidden magnon modes by breaking mirror symmetry, demonstrating control over magnetic excitations via structural distortions and spin-orbit-lattice entanglement.

Contribution

It shows that impurity-induced structural distortions activate symmetry-forbidden magnon modes, advancing understanding of magnetic excitations in spin-orbit-coupled materials.

Findings

Mn substitution reconstructs the magnon spectrum.

Symmetry-forbidden magnon modes become observable.

Polarization dependence indicates lowered rotational symmetry.

Abstract

Correlated materials with competing spin-orbit and crystal-field interactions can host composite spin-orbital magnons that are highly susceptible to structural and electronic perturbations, enabling the control of magnetic dynamics beyond spin-only physics. Using Raman spectroscopy on Ca$_2$RuO$_4$, we show that the partial substitution of Ru with Mn reconstructs the magnon spectrum and reveals one-magnon modes that are hidden in the undoped state. We demonstrate that the transition-metal substitution activates otherwise symmetry-forbidden magnon modes through mirror-symmetry breaking of the underlying spin-orbital configuration. This effect can be theoretically explained by the local structural distortions induced in the RuO$_6$ octahedra near the dopant, that enable the observation of mixed-parity one-magnon modes. These excitations display a distinctive polarization dependence, with a lowering from fourfold to twofold rotational symmetry arising from the mixed-parity character of the coupled magnons and interference between resonant and nonresonant scattering channels. Our results show that spin-orbit-lattice entanglement provides a route to tailoring collective magnetic excitations and their polarization response in spin-orbit-coupled correlated systems.