Q-DEPENDENCE OF SELF-ENERGY EFFECTS OF THE PLANE OXYGEN VIBRATION IN YBA2CU3O7

We have measured the temperature dependence of the peak position and l inewidth of the 42.5 meV phonon branch in a twinned single crystal of YBa2Cu3O7 as a function of wave vector q. In the (100)/(010) direction in the Brillouin zone, considerable softening and broadening occur be low the superconducting transition temperature T-c at some values of q . We observe an order of magnitude smaller softening and no linewidth broadening for q in the (110)/(1(1) over bar0$) direction. Possible im plications of these findings for the symmetry of the superconducting o rder parameter are discussed.