Raman spectroscopic signature of Kitaev magnetism and complex spin-lattice coupling in S = 1/2 antiferromagnet SrLaCoNbO$_6$ double perovskite

Abstract

We report a detailed analysis of temperature-dependent Raman spectroscopy and Co $K$-edge extended x-ray absorption fine structure (EXAFS) for a pseudospin-$\tilde{S}=1/2$ insulating antiferromagnet SrLaCoNbO$_6$, a B-site--ordered double perovskite hosting Co$^{2+}$ ($3d^7$) ions on an {\it f.c.c.} sublattice. Notably, pronounced anomalies in the phonon frequency, linewidth and spectral weight are observed around 60~K, well above the long-range antiferromagnetic transition at $T_{\rm N} \approx 15$~K. These renormalizations indicate a significant coupling between lattice and spin degrees of freedom, although a purely structural contribution cannot be excluded. Additional modifications of both high- and low-energy Raman modes are detected near 160-180 K, including changes in linewidth and intensity, variations of the Fano asymmetry parameter, and the emergence of an additional low-energy feature. The asymmetric Fano line shape of selected low-energy modes, together with a broad low-energy continuum and quasielastic response, suggests coupling between discrete phonons and fluctuating magnetic excitations. Moreover, the EXAFS analysis reveals correlated changes in bond distances and Debye-Waller factors around the Co ions near 60~K and 160~K, evidencing subtle local structural distortions and possible magnetostrictive effects. The persistence of anomalous lattice dynamics far above $T_{\rm N}$, combined with the excitation-energy--independent continuum, is consistent with fluctuating bond-directional interactions and proximate Kitaev-like correlations.