Chirality of matter shows up via spin excitations

• Published in: Nature physics 2012-10, Vol.8 (10), p.734-738
• Main Authors: Bordács, S., Kézsmárki, I., Szaller, D., Demkó, L., Kida, N., Murakawa, H., Onose, Y., Shimano, R., Rõõm, T., Nagel, U., Miyahara, S., Furukawa, N., Tokura, Y.
• Format: Article
• Language: English
• Subjects: 639/301/119/996; 639/766/119/997; Atomic; Chirality; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Crystals; Electronics; Excitation; Ferroelectrics; letter; Magnetic fields; Materials science; Mathematical and Computational Physics; Molecular; Optical activity; Optical and Plasma Physics; Physics; Physics and Astronomy; Theoretical; Translations
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys2387

Chirality is usually manifested by differences in a material’s response to left- and right-circularly polarized light. This difference is the result of the specific distribution of charge within chiral materials. A similar response has now been found to result from the chiral spin structure of an antiferromagnet. An object is considered chiral if its mirror image cannot be brought to coincide with itself by any sequence of simple rotations and translations 1 . Chirality on a microscopic scale—in molecules 2 , 3 , clusters 4 , crystals 5 and metamaterials 6 , 7 —can be detected by differences in the optical response of a substance to right- and left-handed circularly polarized light 2 , 3 . Such ‘optical activity’ is generally considered to be a consequence of the specific distribution of electronic charge within chiral materials. Here, we demonstrate that a similar response can also arise as a result of spin excitations in a magnetic material. Besides this spin-mediated optical activity (SOA), we observe notable differences in the response of Ba 2 CoGe 2 O 7 —a square-lattice antiferromagnet that undergoes a magnetic-field driven transition to a chiral form—to terahertz radiation travelling parallel or antiparallel to an applied magnetic field. At certain frequencies the strength of this magneto-chiral effect 8 , 9 , 10 is almost complete, with the difference between parallel and antiparallel absorption of the material approaching 100%. We attribute these phenomena to the magnetoelectric 11 , 12 nature of spin excitations as they interact with the electric and magnetic components of light.