Infrared study of RBa2Cu4O8 (R=Y,Er,Dy,Gd)

Temperature-dependent far-infrared reflectivity of rare-earth substituted ceramic RBa2Cu4O8 with R=Y, Er, Dy, and Gd is reported. The same number of modes are observed in the rare-earth substituted compounds as in previously reported YBa2Cu4O8 and, except for the Y--rare-earth mode, the shifts in the mode frequencies are small. The temperature-dependent behavior of the phonons and the electronic background in all the samples is also similar to earlier reports of YBa2Cu4O8. Modes near 310 and 500 cm-1 soften significantly with decreasing temperature beginning 30 or 40 K above the zero-resistance Tc. Other modes display little sensitivity to temperature. There is an enhancement in the reflectivity in the superconducting state relative to the normal state for frequencies less than 600 cm-1 that grows in magnitude rapidly below Tc. The spectral range of the enhancement is independent of temperature, rare-earth substitution, and Tc, a behavior that cannot be described by weak-coupling BCS theory