Q-DEPENDENT NEUTRON-SCATTERING - A SIGNATURE OF THE GAP ANISOTROPY INHIGH-T-C SUPERCONDUCTORS

The influence of the superconducting gap anisotropy on the phonon disp ersion is discussed for tetragonal and orthorhombic crystals. Supercon ducting order parameters with different types of s-wave, as well as d- and (s + d)-wave anisotropies, are studied. We show that along specif ic directions of the Brillouin zone the lineshape (energy and linewidt h) of phonons strongly depends on the anisotropy of the gap. Tn this c ontext, we discuss recent experimental results performed on YBa2Cu3O7 single crystals. The data can be well described in terms of an anisotr opic s-wave gap function obtained from the nontotally screened interac tion of charge carriers with long-range optical phonons. The calculati ons suggest, furthermore, that it is generally insufficient to describ e experiments that involve the anisotropy of the superconducting order parameter solely in terms of its symmetry. Not only the symmetry, but also the specific structure of the gap as a function of k is importan t. This conclusion holds for those experiments that are not restricted to the pure determination of the phase of the gap.