Do phonons carry spin?

TL;DR

This paper critically examines the concept of phonon spin, arguing that phonon angular momentum should be interpreted as orbital, and discusses alternative excitations and sources of angular momentum in materials.

Contribution

It provides a fundamental analysis showing phonon spin is physically untenable and introduces the concept of Cosseratons as alternative lattice excitations with spin.

Findings

Phonon spin hypothesis is physically untenable.

Phonon angular momentum should be interpreted as orbital.

Introduction of Cosseratons as lattice excitations with spin.

Abstract

Recently the question of phonon angular momentum has been raised : phonon angular momentum in a magnetic crystal could play an important role in the angular momentum balance in the Einstein-de Haas effect. This proposition is quite natural and understandable, however the idea of phonon spin seems ambiguous in the literature. In 1988 McLellan presented phonon angular momentum states for circular and elliptical polarizations of phonons that represent orbital angular momentum and also pointed out analogy to isotropic quantum harmonic oscillator. In view of its fundamental importance we investigate this issue and conclude that phonon spin hypothesis is physically untenable, and the phonon angular momentum has to be interpreted as orbital. The photon-phonon analogy is also misleading. It is argued that Cosserat elastic medium having internal torque may have lattice excitations different from phonons having spin one or spin two that we term as Cosseratons. In addition a new source of angular momentum balance originationg from anomalous electron magnetic moment is suggested.