In-plane Isotropy of the Low Energy Phonon Anomalies in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+</sub><i><sub>x</sub></i>
In-plane Isotropy of the Low Energy Phonon Anomalies in YBa<sub>2</sub>Cu<sub>3</sub>O<sub>6+</sub><i><sub>x</sub></i>
We study the temperature dependence of the low energy phonons in the $(H, 0, L)$ reciprocal plane of the highly ordered ortho-II YBa$_2$Cu$_3$O$_{6.55}$ cuprate high temperature superconductor by means of high-resolution inelastic x-ray scattering. Anomalies associated with the emergence of long-range charge density wave (CDW) fluctuations are observed, and are …