Chirality Selective Spin-Orbit Exciton-Phonon Coupling in Honeycomb Cobaltates*

The spin-orbit interaction plays a pivotal role in exotic physics such as topologically nontrivial states, unconventional superconductivity and quantum spin liquids. In transition metal compounds with strong spin-orbit coupling (SOC), the SOC can have a significant impact on low-energy magnetic properties, including magnons and spin-orbital excitations. While 3d cobaltates hosts small spin-orbit coupling strength compared to 4d ruthenates and 5d iridates, the entanglement of spin and orbital degrees of freedom is robust. Here, we report the intricate coupling between spin-orbit excitons and phonons in the honeycomb cobaltate CoTiO₃. Using polarized Raman spectroscopy measurements, we identify a chirality selective coupling of spin-orbit excitons and phonons. We find the spin-orbital exchange interactions are directly modulated by lattice vibrations. The spin-orbital-lattice coupling provides insight into the origin of exchange anisotropy, magnetostriction, and magnetodielectric effect in 3d transition metal compounds.