Optical conductivity and infrared-active phonons in RuSr₂GdCu₂O₈

The reflection spectra of polycrystalline RuSr₂GdCu₂O₈ are reported in the frequency range 25-5000 cm^-1 and temperatures between 10 and 300 K. The material exhibits a metallic behavior over the whole temperature range. No superconductivity related features were detected in the low frequency infrared response despite the observed zero resistance below T_c=32 K. A rather low reflection level at frequencies above 800 cm^-1 suggests a strongly overdamped free charge carrier response. Six of the seven A_2u phonon modes, predicted by group theoretical analysis, were observed experimentally and assigned to the definite c-axis eigenmodes by a comparison of the experimental frequencies with the results of lattice dynamical calculations. One of the CuO₂ plane-oxygen related modes exhibits pronounced anomalous softening upon decreasing temperature in the normal state.