q-Dependent phonon renormalization: Possible test for the symmetry of the superconducting gap by neutron scattering?

The q dependence of the phonon renormalization below T-c is studied for superconducting gaps with different symmetries. It is shown that for small to intermediate values of q the angular dependence of the renormalization is characteristic of the gap anisotropy. However, we demonstrate that this dependence is not solely due to the symmetry of the gap, but also to the specific functional form of Delta(k). We compare our calculations with recent neutron scattering experiments done on YBa2Cu3O7. These data are well described in terms of an s-wave anisotropic gap resulting from the nontotally screened interaction of holes with long-wave, optical phonons.