Spin Wave Excitations in the Multiferroic Ba$_2$CoGe$_2$O$_7$

ORAL

Abstract

Ba$_2$CoGe$_2$O$_7$ is a multiferroic material where spin waves exhibit giant directional dichroism and natural optical activity at THz frequencies due to the large ac magnetoelectric effect [S. Bordacs et al., Nature Physics {\bf 8}, 734 (2012)]. We studied spin excitations in the magnetically ordered phase of the noncentrosymmetric Ba$_2$CoGe$_2$O$_7$ in high magnetic fields up to 33 T [Penc et al., Phys. Rev. Lett. {\bf 108}, 257203 (2012)]. In the ESR and THz absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. A multiboson spin-wave theory describes these unconventional modes, including spin-stretching modes, characterized by an oscillating magnetic dipole and quadrupole moment. The lack of inversion symmetry allows each mode to become electric dipole active.

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Authors

  • Toomas Room

    • National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

  • Karlo Penc

    • Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest

  • Judit Romhanyi

    • Institute for Solid State Physics and Optics, Hungarian Academy of Sciences, Budapest

  • Urmas Nagel

    • Nat.-l Inst. of Chem. Phys. \& Biophys., Tallinn, Estonia
    • National Institute of Chemical Physics and Biophysics, Tallinn, Estonia

  • Agnes Antal

    • Department of Physics, Budapest University of Technology, Hungary

  • Titus Feher

    • Department of Physics, Budapest University of Technology, Hungary

  • Andras Janossy

    • Department of Physics, Budapest University of Technology, Hungary

  • Hans Engelkamp

    • High Field Magnet Laboratory, Institute for Molecules and Materials, Radboud University, Nijmegen, The Netherlands

  • H. Murakawa

    • Quantum-Phase Electronics Center, Department of Applied Physics, The University of Tokyo, Japan

  • Y. Tokura

    • CERG, RIKEN ASI
    • Quantum-Phase Electronics Center, Department of Applied Physics, The University of Tokyo, Japan
    • Department of Applied Physics, University of Tokyo

  • David Szaller

    • Department of Physics, Budapest University of Technology, Hungary

  • Sandor Bordacs

    • Department of Physics, Budapest University of Technology, Hungary

  • Istvan Kezsmarki

    • Department of Physics, Budapest University of Technology, Hungary