Chirality of Matter Shows Up via Spin Excitations

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.

References in corpus (5)

• Electric-field control of spin waves at room temperature in multiferroic BiFeO3
• Ferroelectricity induced by spin-dependent metal-ligand hybridization in Ba${}_2$CoGe${}_2$O${}_7$
• Giant directional dichroism of terahertz light in resonance with magnetic excitations of the multiferroic oxide BaCo${}_2$Ge${}_2$O${}_7$
• Terahertz time-domain spectroscopy of electromagnons in multiferroic perovskite manganites
• Theory of magnetoelectric resonance in two-dimensional $S=3/2$ antiferromagnet $\mathrm{B}{\mathrm{a}}_2\mathrm{CoG}{\mathrm{e}}_2{\mathrm{O}}_7$ via spin-dependent metal-ligand hybridization mechanism

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