Temperature-Dependent Infrared Reflectivity Studies of Multiferroic TbMnO_{3}: Evidence for Spin-Phonon Coupling

Abstract

We have measured near normal incidence far infrared (FIR) reflectivity spectra of a single crystal of TbMnO3 from 10K to 300K in the spectral range of 50 cm${}^{-1}$ to 700 cm${}^{-1}$. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function $\hat{I}{\mu }_2$($\ddot{I}‰$) and energy loss function Im(-1/$\hat{I}\mu$($\ddot{I}‰$)), respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature T${}_N$ (~ 46K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the super-exchange integral between the Mn${}^{3+}$ spins. The effective charge of oxygen (Z${}_O$) calculated using the measured LO-TO splitting increases below T${}_N$.

16 pages, 6 figures, 1 table