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.

Figures (3)

• Figure 1: (a) Temperature-dependent Raman spectra of Ca$_2$(Ru,Mn)O$_4$ with 0%, 3%, 5%, and 10% Mn content measured in the $B_{1g}$ symmetry channel. Dashed lines mark the Néel temperatures. The arrows indicate one-magnon excitations. (b) Individual Raman spectra measured at $T = 4$ K. One-magnon contributions are shaded. The black asterisks mark a phonon that partially overlaps with magnons. (c) Magnon energies and (d) their integrated intensities as a function of Mn content.
• Figure 2: (a) Schematic representation of the of the three lowest-lying local spin--orbital states stabilized by SOC and octahedral distortions in Ca$_{2}$RuO$_{4}$. The tilting of the RuO$_6$ octahedra with respect to the $b$ axis mixes the orbital characters and splits the multiplets into states of definite parity with respect to the diagonal vertical mirror (the vertical plane $ac$ perpendicular to the $b$ axis). The lowest state in energy has odd parity, while the higher-energy doublet consists of states with opposite parity, even/odd. The panel illustrates a representative case for distortions associated with a specific sign of the parameter $\delta_c$ as related to the staggering of the octahedral rotations with respect to the $c$-axis; for the opposite sign, the parity of these two states is inverted. (b) and (c) schematically depict the evolution of the magnon spectrum for the antiferromagnetic state from the undoped to the Mn-doped (orange) configuration with the substitution being a source of diagonal vertical mirror ($\mathbb{M}$) symmetry breaking. The breaking of the vertical mirror activates a one-magnon mode in the cross polarization channel that is silent in the Ca$_2$RuO$_4$ compound (c). Here, we focus on the excitations of the Ru magnetic moments, with the oxygen degrees of freedom projected out. This enables the application of a mirror transformation to the Ru lattice and the associated spin configuration, as schematically illustrated in panel (b).
• Figure 3: Raman spectral function evaluated for a $4\times4$ cluster with periodic boundary conditions and in the presence of inhomogeneous anisotropic magnetic energy $E_T$ nearby the substituted site (corresponding to the darkest orange square in the schematic cluster). The parameters of the model are $J_{xx}=5.2$, $J_{xy}=3.1$, $J_{z}=2$, $E_T=30$ (in units of meV) at host sites. At the impurity sites the anisotropic magnetic energy $E_T$ is modified to acquire the value $E_T=40$ and is also varied at the neighboring sites of the impurity, as illustrated by the contour map. For clarity we employ Lorentzian distributions for the spectral function with broadening factor $\eta=0.06$ meV.