Type: Article
Publication Date: 2011-10-19
Citations: 36
DOI: https://doi.org/10.1103/physrevlett.107.177004
Motivated by predictions of a substantial contribution of the ``buckling'' vibration of the ${\mathrm{CuO}}_{2}$ 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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7}$. The data reveal a pronounced softening of the phonon at the in-plane wave vector $\mathbf{q}=(0,0.3)$ upon cooling below $\ensuremath{\sim}105\text{ }\text{ }\mathrm{K}$, but no corresponding anomaly at $\mathbf{q}=(0.3,0)$. 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.