Temperature-dependent infrared reflectivity studies of multiferroic TbMnO₃: evidence for spin-phonon coupling

Abstract

We have measured near normal incidence far-infrared (FIR) reflectivity spectra of a single crystal of TbMnO₃ from 10 K to 300 K in the spectral range of 50 cm⁻¹−700 cm⁻¹. Fifteen transverse optic (TO) and longitudinal optic (LO) modes are identified in the imaginary part of the dielectric function ε₂(ω) and energy loss function Im(−1/ε(ω)), respectively. Some of the observed phonon modes show anomalous softening below the magnetic transition temperature T_N (~46 K). We attribute this anomalous softening to the spin-phonon coupling caused by phonon modulation of the superexchange integral between the Mn³⁺ spins. The effective charge of oxygen (ZO) calculated using the measured LO-TO splitting increases below T_N.