Magnon-phonon conversion experiment and phonon spin

TL;DR

This paper critically analyzes a recent experiment claiming phonon spin observation, arguing that the data actually reflects angular momentum exchange consistent with phonon orbital angular momentum rather than spin.

Contribution

It clarifies the distinction between phonon spin and orbital angular momentum, challenging the interpretation of experimental data as evidence of phonon spin.

Findings

Experimental data shows angular momentum exchange of ±ħ per photon.

The observed effects are attributed to phonon orbital angular momentum.

Phonon spin hypothesis is argued to be unphysical.

Abstract

Recent experiment demonstrates magnon to phonon conversion in a YIG film under the application of a non-uniform magnetic field. Light scattered from phonons is observed to change its polarization state interpreted by the authors signifying phonon spin. In this note we argue that the experimental data merely shows the exchange of angular momentum $\pm \hbar$ per photon. We suggest that it has physical origin in the orbital angular momentum of phonons. The distinction between spin and orbital parts of the total angular momentum, and between phonons and photons with added emphasis on their polarizations is explained. The main conclusion of the present note is that phonon spin hypothesis is unphysical.