Antiferro-Chiral Phonons in $\mathcal{P}\mathcal{T}$-Symmetric Antiferromagnets

Abstract

Chiral phonons provide a route to couple lattice motion to magnetic order, but conventional chiral phonons carry a net angular momentum and thus couple naturally to net magnetization rather than to compensated N\'eel order. Here we show that $\mathcal{P}\mathcal{T}$-symmetric antiferromagnets can host \emph{antiferro-chiral phonons} (AFCPs): phonon modes with vanishing total angular momentum but finite sublattice-staggered angular momentum. Symmetry enforces this distinction because $\mathcal{P}\mathcal{T}$ forbids a net phonon angular momentum while allowing counter-rotating local motion on inversion-related sublattices. AFCPs arise from a N\'eel-vector-locked coupling between Raman and infrared-active phonons. The coupling is odd under both $\mathcal{P}$ and $\mathcal{T}$ while preserving their product. Through this hybridization, the normal modes acquire both Raman and infrared character and carry a sublattice-staggered phonon angular momentum that acts as a conjugate field to the N\'eel vector. This coupling is microscopically generated by the molecular Berry curvature, which is demonstrated in a prototype lattice model. Reversing the N\'eel vector reverses the staggered phonon chirality. These results indicate AFCPs as probes of antiferromagnetic order and suggest coherent phonon excitation as a route to its dynamical control.