We provide a detailed study of the reflectivity of multiferroic TbMnO_3 for wave numbers from 40 cm^{-1} to 1000 cm^{-1} and temperatures 5 K < T < 300 K. Excitations are studied for polarization directions E || a, the polarization where electromagnons are observed, and for E || c, the direction of the spontaneous polarization in this material. The temperature dependencies of eigenfrequencies, damping constants and polar strengths of all modes are studied and analyzed. For E || a and below the spiral ordering temperature of about 27 K we observe a transfer of optical weight from phonon excitations to electromagnons, which mainly involves low-frequency phonons. For E || c an unusual increase of the total polar strength and hence of the dielectric constant is observed indicating significant transfer of dynamic charge probably within manganese-oxygen bonds on decreasing temperatures.
@article{arxiv.0905.2921,
title = {Far-infrared optical excitations in multiferroic TbMnO_3},
author = {M. Schmidt and Ch. Kant and T. Rudolf and F. Mayr and A. A. Mukhin and A. M. Balbashov and J. Deisenhofer and A. Loidl},
journal= {arXiv preprint arXiv:0905.2921},
year = {2010}
}
Comments
8 pages, 7 figures, accepted for submission in European Physical Journal B