Highly anisotropic anomaly in the dispersion of the copper-oxygen bond-bending phonon in superconducting YBa₂Cu₃O ₇ from inelastic neutron scattering

Motivated by predictions of a substantial contribution of the "buckling" vibration of the CuO₂ layers to d-wave superconductivity in the cuprates, we have performed an inelastic neutron scattering study of this phonon in an array of untwinned crystals of YBa ₂Cu₃O₇. The data reveal a pronounced softening of the phonon at the in-plane wave vector q=(0,0.3) upon cooling below ∼105K, but no corresponding anomaly at q=(0.3,0). Based on the observed in-plane anisotropy, we argue that the electron-phonon interaction responsible for this anomaly supports an electronic instability associated with a uniaxial charge-density modulation and does not mediate d-wave superconductivity.