In crystals with broken time-reversal symmetry, zone-center phonons can acquire a finite angular momentum via velocity-dependent forces on the nuclei. Despite having the same order of magnitude as the electron spin angular momentum, the phonon angular momentum can be hard to detect because the frequency splitting is small. Here, by developing a theory of lattice magneto-optical effects in reflection and transmission, we show that infrared magnetic circular dichroism is a sensitive probe of zone-center phonon chirality. We evaluate the infrared magneto-optical Faraday, Kerr, and circular-dichroism spectra of CrI3 from time-dependent density-functional theory in the adiabatic local-density approximation. We find sizeable circular dichroism from the infrared-active Eu mode at ≈214 cm−1, even though the calculated splitting is only 0.22 cm−1.
@article{arxiv.2506.11258,
title = {Theory of infrared magneto-optical effects from chiral phonons in solids},
author = {Chiara Fiorazzo and Cheol-Hwan Park and Ivo Souza and Matteo Calandra},
journal= {arXiv preprint arXiv:2506.11258},
year = {2025}
}