title: Spin-stretching modes in anisotropic magnets: spin-wave excitations in the multiferroic Ba2CoGe2O7.
abstract: 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. In the electron spin resonance and far infrared absorption spectra we found several spin excitations beyond the two conventional magnon modes expected for such a two-sublattice antiferromagnet. We show that 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. We expect that the spin-stretching modes can be generally observed in inelastic neutron scattering and light absorption experiments in a broad class of ordered S > 1/2 spin systems with strong single-ion anisotropy and/or noncentrosymmetric lattice structure.
authors.affiliation: Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, H-1525 Budapest, POB 49, Hungary.
authors.forename: K
authors.initials: K
authors.lastname: Penc
authors.forename: J
authors.initials: J
authors.lastname: Romhányi
authors.forename: T
authors.initials: T
authors.lastname: Rõõm
authors.forename: U
authors.initials: U
authors.lastname: Nagel
authors.forename: A
authors.initials: A
authors.lastname: Antal
authors.forename: T
authors.initials: T
authors.lastname: Fehér
authors.forename: A
authors.initials: A
authors.lastname: Jánossy
authors.forename: H
authors.initials: H
authors.lastname: Engelkamp
authors.forename: H
authors.initials: H
authors.lastname: Murakawa
authors.forename: Y
authors.initials: Y
authors.lastname: Tokura
authors.forename: D
authors.initials: D
authors.lastname: Szaller
authors.forename: S
authors.initials: S
authors.lastname: Bordács
authors.forename: I
authors.initials: I
authors.lastname: Kézsmárki