q dependence of self-energy effects of the plane oxygen vibration in YBa2Cu3O7.

We have measured the temperature dependence of the peak position and linewidth of the 42.5 meV phonon branch in a twinned single crystal of YBa 2Cu 3O 7 as a function of wave vector q. In the s100dys010d direction in the Brillouin zone, considerable softening and broadening occur below the superconducting transition temperature Tc at some values of q. We observe an order of magnitude smaller softening and no linewidth broadening for q in the s110dy ° 110 ¢ direction. Possible implications of these findings for the symmetry of the superconducting order parameter are discussed.