Theory of infrared magneto-optical effects from chiral phonons in solids

TL;DR

This paper develops a theoretical framework to detect phonon chirality in solids with broken time-reversal symmetry using infrared magneto-optical effects, demonstrating measurable circular dichroism in CrI3.

Contribution

It introduces a new theory linking phonon angular momentum to magneto-optical effects and applies it to predict observable signals in specific materials.

Findings

Significant circular dichroism observed in CrI3 at infrared frequencies.

Theoretical prediction of measurable optical effects from phonon chirality.

Small phonon frequency splitting can produce detectable magneto-optical signals.

Abstract

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 CrI$_3$ from time-dependent density-functional theory in the adiabatic local-density approximation. We find sizeable circular dichroism from the infrared-active E$_u$ mode at $\approx 214$ cm$^{-1}$, even though the calculated splitting is only 0.22 cm$^{-1}$.