Dopant-Induced Symmetry Breaking Reveals Hidden Magnons in a Spin-Orbit Correlated Material

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 control of magnetic dynamics beyond spin-only physics. Using Raman spectroscopy on Ca$_2$RuO$_4$, we show that the partial substitution of Ru by Mn reconstructs the magnon spectrum and leads to 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. The uncovered mechanism demonstrates how spin-orbit-lattice entanglement can be exploited to control collective magnetic excitations in spin-orbit correlated materials.