Temperature (5--250 K) and magnetic field (0--70 kOe) variations of the
low-energy (1--10 meV) electrodynamics of spin excitations have been
investigated for a complete set of light-polarization configurations for a
ferroelectric magnet DyMnO3​ by using terahertz time-domain spectroscopy. We
identify the pronounced absorption continuum (1--8 meV) with a peak feature
around 2 meV, which is electric-dipole active only for the light E-vector
along the a-axis. This absorption band grows in intensity with lowering
temperature from the spin-collinear paraelectric phase above the ferroelectric
transition, but is independent of the orientation of spiral spin plane (bc or
ab), as shown on the original Ps​ (ferroelectric polarization)
∥c phase as well as the magnetic field induced Ps​∥a phase. The possible origin of this electric-dipole active band is argued in
terms of the large fluctuations of spins and spin-current.Comment: New version, 11 pages including colored 8 figure