Gap Anisotropy and Phonon Renormalization in HTS*

Anisotropy of the order parameter (gap) in high-T-c superconductors is explained by unscreened interaction of charge carriers with long-wave optical phonons. Screening is absent due to the low frequency of long-wave plasmons in layered structures. Numerical calculations of the gap anisotropy caused by the unscreened interaction show good agreement of the model with the experiment. The influence of the superconducting gap anisotropy on the phonon dispersion is discussed. It is found that along specific directions of the Brillouin zone the lineshape (energy and linewidth) of phonons depends strongly on the type of anisotropy of the gap. In this context, the recent experimental results performed on YBa2Cu3O7 single crystals are discussed. The data can be well described in terms of the anisotropic gap function obtained from the non-totally screened interaction of charge carriers with long-wave optical phonons.