Far-infrared optical excitations in multiferroic TbMnO_3

TL;DR

This study investigates the infrared optical properties of multiferroic TbMnO_3, revealing temperature-dependent excitations, electromagnon activity, and charge transfer phenomena related to its multiferroic behavior.

Contribution

It provides a detailed analysis of the temperature-dependent optical excitations and electromagnons in TbMnO_3, highlighting new insights into its multiferroic properties.

Findings

Transfer of optical weight from phonons to electromagnons below 27 K.

Unusual increase in dielectric constant along c-axis with decreasing temperature.

Significant dynamic charge transfer within manganese-oxygen bonds.

Abstract

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.