Phonon anomalies and range of superconducting energy gaps from infrared studies of YBa2Cu3O7- delta.

The infrared reflectivity of sintered YBa/sub 2/Cu/sub 3/O/sub 7/minus//delta//, prepared under carefully controlled oxidation conditions, has been measured between 20 and 10 000 cm/sup /minus/1/ at temperatures between 10 and 300 K. In this paper we analyze in detail the temperature- and frequency-dependent far-infrared part of the spectrum. A Kramers-Kronig analysis allows us to compare the phonon part of the spectra with lattice-dynamical calculations. There is fairly good agreement concerning the assigned eigenfrequencies. The oscillator strengths of the experimentally observed phonons, however, are all stronger than those from a lattice-dynamics model which assumes ionic charges close to the formal ones. This is especially so for the lowest phonon found at 155 cm/sup /minus/1/ which shows a strength /ital S/=60/plus minus/5 being about 15 times higher than theoretically expected. A possible origin of this effect is discussed qualitatively in terms of a Cu(1)-O(IV) charge shift during the vibration induced by the polarization field. The electronic background of the spectra has been analyzed by a Mattis-Bardeen type of dielectric response below /ital T//sub /ital c//. The best fit yielded, for low /ital T/, a gap distribution in the range from about 160 to 370 cm/sup /minus/1/, corresponding to 2/Delta/(0)//ital k//sub /ital B//T/submore » c/ values between 2.6 and 6.0.« less