Disorder-Induced Slow Relaxation of Phonon Polarization

Circularly polarized phonons, which carry angular momentum, have attracted considerable interest because of their coupling to magnetism. However, the relaxation time of their circular polarization—which governs their generation efficiency and diffusion length—has been challenging to describe within conventional Boltzmann theory. In this talk, we theoretically clarify the relaxation mechanisms of both linear and circular polarizations of acoustic phonons by employing the phonon density matrix and quantum kinetic equation. Our analysis shows that under scattering by isotropic defects, frequent collisions lead to slower relaxation of phonon polarization—opposite to the trend in momentum relaxation. This mechanism is analogous to the Dyakonov-Perel spin relaxation in semiconductors and to motional narrowing in NMR. We derive analytical expressions for the relaxation times in isotropic elastic media and perform numerical calculations for cubic crystals. These results suggest a route to prolong the lifetime of phonon angular momentum in solids.