On the effect of the superconducting phase transition on phonons in YBa2Cu4O8: an infrared spectroscopic study

Phonon spectra of YBa 2 Cu 4 O 8 have been measured in the range 110-1000 cm -1 and between 30 K and room temperature. All seven B 1u -symmetry infrared-active phonons are observed at 119, 135, 200, 314, 332, 500 and 605 cm -1 at 290 K. The effect of decreasing temperature on phonons with frequencies >or approximately= 210 cm -1 has been studied and it is found that the 500, 314 and 605 cm -1 modes-and perhaps also the 332 cm -1 mode-exhibit changes in frequency, intensity and linewidth at the superconducting transition temperature (80 K). The observed phonon renormalization effects are compared to those previously observed in YBa 2 Cu 3 O 7- delta . The authors' results suggest that polaron-phonon interactions may be important for the occurrence of superconductivity in these materials-e.g. via the Bose condensation of bipolarons.