SUPERCONDUCTING GAP IN PRXY1-XBA2CU4O8 AND YBA2-YSRYCU4O8 PROBED BY INFRARED PHONON SELF-ENERGIES

We report a reflectivity study of the z-polarized TO-phonons of PrxY1- xBa2Cu4O8 and YBa2-ySryCu4O8 alloys in the temperature range 10-300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibratio n at omega approximately 300 cm-1 due to the opening of the supercondu cting gap are found to occur upon crossing the superconducting transit ion temperature T(c). Phonon self-energy effects are strongly dependen t on T(c), providing evidence for a relative shift of the gap with res pect to the energy of phonon.