Extrinsic Mechanisms of Phonon Magnetic Moment

We develop a general formalism of the phonon magnetic moment by including the relaxation processes. We then identify the skew-scattering and side-jump contributions to the phonon magnetic moment originating from the nonadiabaticity, both of which are related to the nonlocal phonon Berry curvature and are in close analogy to those in the electronic Hall effect. All contributions of the phonon magnetic moment are exemplified in a honeycomb lattice, showing that the extrinsic contribution can be as important as the intrinsic one and that the resulting phonon angular momentum varies significantly across the Brillouin zone. Our Letter offers a systematic framework of the phonon chirality and paves the way for tuning the phonon magnetic moment through the nonadiabaticity.