Far - infrared optical excitations in multiferroic TbMnO 3

We provide a detailed study of the reflectivity of multiferroic TbMnO3 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.