Nonreciprocal phonons in PT-symmetric antiferromagnet

TL;DR

This paper demonstrates that nonreciprocal phonons can exist in PT-symmetric antiferromagnets without magnetic fields, using phenomenological theory and spin-orbit coupling to control and detect the effect.

Contribution

It introduces the concepts of flexo-viscosity and flexo-torque as mechanisms for phonon nonreciprocity in antiferromagnets, linking symmetry breaking to phonon transport.

Findings

Nonreciprocal phonons exist without magnetic fields in PT-symmetric antiferromagnets.

Spin-orbit coupling enables electrical control of phonon nonreciprocity.

Phonon nonreciprocity correlates with the Neel vector, indicating the order parameter.

Abstract

Phonon nonreciprocity, indicating different transport properties along opposite directions, has been observed in experiments under a magnetic field. We show that nonreciprocal acoustic phonons can also exist without a magnetic field nor net magnetization. We focus on PT symmetric antiferromagnets that break both time-reversal T and inversion symmetry P. We identify crucial contributions in phenomenological elastic theory, dubbed flexo-viscosity and flexo-torque, that induce phonon nonreciprocity without changing the phonon polarization. The microscopic origin of these contributions is the molecular Berry curvature, manifested as emergent nonlocal magnetic fields on phonons. The symmetry breaking originated from spin order is transferred to the phonon system through spin-orbit coupling, where the orbital degree of freedom affects the lattice dynamics directly. By electrically modifying the spin-orbit coupling, we show that both the phonon nonreciprocity and helicity can be controlled and enhanced. Importantly, the phonon nonreciprocity is an odd function of the N\'eel vector, serving as an indicator of the order parameter.