Temperature dependence of the bond-stretching phonon anomaly in YBa2Cu3O6.95

We report the results of a detailed inelastic neutron-scattering study of renormalization of the bond-stretching phonons in optimally doped YBa2Cu3O6+x (YBCO). In agreement with previous work, this renormalization is strongest halfway to the zone boundary in the [010] direction where it manifests itself as a transfer of phonon spectral weight from similar to 60 to similar to 50 meV. Phonon renormalization begins on cooling from well above the superconducting transition temperature, T-c, but strongly accelerates at T-c. In contrast with the La2-xSrxCuO4 system, where a similar longitudinal phonon anomaly occurs, the anomaly in YBCO appears to extend in the transverse direction along the branch that connects the longitudinal mode at the in-plane wave vector q(in)= (0, 0.25) and the transverse bond-stretching mode at q(in)= (0.25, 0.25). Therefore, phonon renormalization in YBa2Cu3O6.95 seems to be consistent with a quasi-one-dimensional behavior.