Anti-Chiral Phonons: Sublattice Angular Momentum in PT Symmetric Systems

Phonons govern a material’s thermal, mechanical, and quantum behaviors. Among them, chiral phonons carrying phonon angular momentum have attracted intense interest for their interplay with magnetic fields, spins, and orbital magnetism. They have been theoretically proposed in gapped graphene, where C₃ symmetry protects phonons with nonzero angular momentum at the K and K' valleys, and experimentally observed in materials such as monolayer WSe₂, α-HgS, quartz, and LiNbO₃. Yet a fundamental question remains: can phonons exhibit antichirality? Here we introduce anti-chiral phonons (ACPs), modes with zero total angular momentum but opposite circular motion on distinct sublattices. We show that ACPs generally emerge in materials with PT symmetry at momenta away from time-reversal invariant points. The antichirality can be stabilized by molecular Berry curvature, with antichirality locked to magnetic order. Unlike conventional chiral phonons, ACPs possess sublattice-resolved angular momentum, enabling coupling to the order parameter of antiferromagnetic systems. These results establish a new pathway for phonon-driven spin manipulation in quantum materials.