Chirality of Matter Shows Up via Spin Excitations

TL;DR

This paper demonstrates that spin excitations in magnetic materials can exhibit optical activity and magnetochiral effects, revealing a new form of matter chirality linked to spin dynamics and external magnetic fields.

Contribution

It introduces the concept that spin excitations can show optical activity in chiral magnetic states, expanding the understanding of chirality beyond electronic charges.

Findings

Large optical activity observed at spin resonance frequencies.

Magnetochiral effect with nearly 100% absorption difference.

Chirality of spin excitations induced by magnetic fields.

Abstract

Right- and left-handed circularly polarized light interact differently with electronic charges in chiral materials. This asymmetry generates the natural circular dichroism and gyrotropy, also known as the optical activity. Here we demonstrate that optical activity is not a privilege of the electronic charge excitations but it can also emerge for the spin excitations in magnetic matter. The square-lattice antiferromagnet Ba$_2$CoGe$_2$O$_7$ offers an ideal arena to test this idea, since it can be transformed to a chiral form by application of external magnetic fields. As a direct proof of the field-induced chiral state, we observed large optical activity when the light is in resonance with spin excitations at sub-terahertz frequencies. In addition, we found that the magnetochiral effect, the absorption difference for the light beams propagating parallel and anti-parallel to the applied magnetic field, has an exceptionally large amplitude close to 100%. All these features are ascribed to the magnetoelectric nature of spin excitations as they interact both with the electric and magnetic components of light.