Disorder-Induced Slow Relaxation of Phonon Polarization

TL;DR

This paper uncovers a counterintuitive phonon polarization relaxation mechanism where increased scattering slows relaxation, potentially extending phonon angular momentum lifetime, with implications for lattice dynamics understanding.

Contribution

It introduces a theoretical relaxation mechanism for phonon polarization, validated through quantum kinetic equations, revealing slower relaxation with more scattering and differences between linear and circular polarizations.

Findings

More frequent scattering causes slower polarization relaxation.

Linear polarizations relax more slowly than circular ones.

The relaxation rates are derived analytically and numerically.

Abstract

The role of the polarization degree of freedom in lattice dynamics in solids has been underlined recently. We theoretically discover a relaxation mechanism for both linear and circular polarizations of acoustic phonons. In the absence of scattering, the polarization exhibits oscillatory behavior. This behavior leads to a counterintuitive result: unlike linear momentum, more frequent scattering events cause slower polarization relaxation due to motional narrowing. We validate this mechanism using the quantum kinetic equation. We derive the relaxation rates of polarizations analytically for isotropic elastic bodies and numerically for a cubic crystal. Remarkably, we reveal that linear polarizations relax more slowly than circular ones. Our findings provide a pathway to extend the lifetime of phonon angular momentum.