Abstract:
We have measured near normal incidence far-infrared (FIR) re°ectivity spec-
tra 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 identi¯ed in
the imaginary part of the dielectric function "2(!) and energy loss function Im(¡1/"(!)),
respectively. Some of the observed phonon modes show anomalous softening below the
magnetic transition temperature TN (»46 K). We attribute this anomalous softening to
the spin-phonon coupling caused by phonon modulation of the superexchange integral be-
tween the Mn3+ spins. The e®ective charge of oxygen (ZO) calculated using the measured LO{TO splitting increases below TN.
