Electromagnons in multiferroic YMn2O5 and TbMn2O5

Based on temperature dependent far infrared transmission spectra of YMn2O5 and TbMn2O5 single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the step-like anomaly of the static dielectric constant at the commensurate--incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.Comment: 4 pages, 4 figures, submitte

Spin phonon coupling in frustrated magnet CdCr2_2O4_4

The infrared phonon spectrum of the spinel CdCr2O4 is measured as a function temperature from 6 K to 300K. The triply degenerate Cr phonons soften in the paramagnetic phase as temperature is lowered below 100 K and then split into a singlet and doublet in the low T antiferromagnetic phase which is tetragonally distorted to relieve the geometric frustration in the pyrochlore lattice of Cr$^{3+}$ ions. The phonon splitting is inconsistent with the simple increase (decrease) in the force constants due to deceasing (increasing) bond lengths in the tetragonal phase. Rather they correspond to changes in the force constants due to the magnetic order in the antiferromagnetic state. The phonon splitting in this system is opposite of that observed earlier in ZnCr2O4 as predicted by theory. The magnitude of the splitting gives a measure of the spin phonon coupling strength which is smaller than in the case of ZnCr2O4.Comment: 4.2 pages, 4 figures, 1 reference added, submmite

A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNb2_2O6_6 investigated by high resolution time-domain terahertz spectroscopy

Kink bound states in the one dimensional ferromagnetic Ising chain compound CoNb$_2$O$_6$ have been studied using high resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr\"odinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near two times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states -- two pairs of kinks on neighboring chains