title: Dynamical magnetoelectric coupling in helical magnets.
abstract: We develop a theory of collective mode dynamics in the helical magnets coupled to electric polarization via spin-orbit interaction. The low-lying modes associated with the ferroelectricity are not the transverse optical phonons, but are the spin waves hybridized with the electric polarization. This hybridization leads to the Drude-like dielectric function epsilon(omega) in the limit of zero magnetic anisotropy. There are two additional low-lying modes: phason of the spiral and rotation of helical plane along the polarization axis. Role of these low-lying modes in the neutron scattering and antiferromagnetic resonance is revealed, and a novel experiment to detect the dynamical magnetoelectric coupling is discussed.
authors.affiliation: Department of Applied Physics, the University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Hosho
authors.initials: H
authors.lastname: Katsura
authors.forename: Alexander V
authors.initials: AV
authors.lastname: Balatsky
authors.forename: Naoto
authors.initials: N
authors.lastname: Nagaosa