Raman spectroscopy of multiferroic TbMnO$_3$

Coupling between the lattice and magnetic degrees of freedom in TbMnO$_3$ has been observed to produce magnetic excitations with electric dipole activity, or electromagnons. Recent reports of electromagnons in other multiferroic (113)-orthomanganites\footnote{R. Vald\'{e}s Aguilar \textit{et al.}, Phys. Rev. B \textbf{76}, 060404 (2007).} and related (125)-manganites\footnote{A. B. Sushkov \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 027202 (2007).} indicate a complementary Raman study may provide additional insight into the importance of spin-lattice coupling. We present Raman spectra of single-crystal and polycrystalline TbMnO$_3$ using a triple-grating spectrometer in a collinear backscattering configuration as a function of temperature ($4-300\,$K) and polarization along various crystallographic axes. The absence of any observable low-frequency modes (intensity $<1000$ times that of prominent Raman-active phonons) suggests a weak scattering cross-section for the electromagnon. Additionally, we discuss the temperature dependence of Raman-active phonons and compare with results from infrared measurements.