Spin-lattice coupling induced chiral phonons and their signature in Raman Circular Dichroism

TL;DR

This paper explains the observed chiral phonons in $ ext{RuCl}_3$'s Raman circular dichroism through a spin-phonon coupling model, showing how magnetic fields induce complex phonon polarization and chiral signatures.

Contribution

It introduces a diagrammatic framework for spin-phonon interactions in the Kitaev model, linking magnetic field effects to chiral phonon signatures in Raman spectra.

Findings

Raman circular dichroism arises from spin-phonon coupling under magnetic fields.

Interaction modifies phonon polarization, creating complex superpositions with finite angular momentum.

The model's predictions match experimental growth of RCD with magnetic field.

Abstract

Recent Raman experiments on the Kitaev material $\alpha$-RuCl$_3$ have reported a finite Raman circular dichroism (RCD), revealing chiral phonon behaviour not expected from lattice symmetry alone. To explain this observation, we develop a diagrammatic framework for the spin-phonon coupled Kitaev model. We demonstrate that bare phonons contribute no RCD, but coupling to the chiral spin excitation continuum under an applied magnetic field renormalizes the phonon propagator, mixing real polarization eigenvectors into complex superpositions with finite angular momentum. This interaction-induced modification generates a nonzero RCD accompanied by characteristic Fano line shapes in the Raman response, reflecting interference between discrete phonons and the continuum. The resulting signal grows with magnetic field strength, consistent with experiment, and directly tracks the field-induced chirality of the spin sector. More broadly, our results establish RCD as a powerful probe of interaction-induced chiral phonons in correlated quantum materials.