Superconducting gap in Pr x Y1−xBa2Cu4O8 and YBa2−ySr y Cu4O8 probed by infrared phonon self-energies

We report a reflectivity study of thez-polarized TO-phonons of Pr x Y1−xBa2Cu4O8 and YBa2−ySr y Cu4O8 alloys in the temperature range 10–300 K. Anomalies of the frequency and linewidth of the plane-oxygen vibration atω∼300 cm−1 due to the opening of the superconducting gap are found to occur upon crossing the superconducting transition temperatureT c . Phonon self-energy effects are strongly dependent onT c , providing evidence for a relative shift of the gap with respect to the energy of phonon.