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

TL;DR

This paper introduces antiferro-chiral phonons in $ ext{PT}$-symmetric antiferromagnets, which have zero net angular momentum but finite sublattice-staggered angular momentum, enabling new ways to probe and control antiferromagnetic order.

Contribution

It demonstrates the existence of antiferro-chiral phonons with staggered angular momentum in $ ext{PT}$-symmetric antiferromagnets and explains their microscopic origin via molecular Berry curvature.

Findings

Antiferro-chiral phonons have zero net but finite staggered angular momentum.

Reversing the Néel vector reverses phonon chirality.

AFCPs can be used to probe and control antiferromagnetic order.

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.