Phonons are sound waves—pressure oscillations in a crystal lattice. They are time-reversal symmetric by default. They carry no angular momentum preference. They do not know about chirality.
Chatterjee and Liu show that in magnetic topological insulators, the surface phonon Hall viscosity—a topological property of the electronic sector—reaches into the lattice and gives phonons circular polarization. The mechanism: Hall viscosity at the surface creates an interface phonon mode whose frequency sits below the bulk mode. This interface mode has a definite circular polarization, and it functions as a polarization filter: only phonons matching that handedness are confined to and transmitted along the interface.
The authors also describe an acoustic Faraday rotation effect and a scattering framework for mode conversion between linear and circular phonon polarizations.
This is a case of topological properties propagating across physical domains. The electrons have nontrivial topology. The phonons, coupled to the electrons through the lattice, inherit chirality they could never generate on their own. The symmetry-breaking that makes magnetic topological insulators interesting lives in the electronic Hamiltonian, but its consequences extend to mechanical vibrations of atoms.
The interface mode is particularly striking: it exists below the bulk phonon frequency, meaning it is not a high-energy excitation but a low-energy consequence of boundary topology. A fundamentally achiral degree of freedom acquires chirality from a fundamentally different subsystem. The electronic topology imposes preferences on the mechanical world.