In-plane Isotropy of the Low Energy Phonon Anomalies in YBa$_{2}$Cu$_{3}$O$_{6+x}$

TL;DR

This study investigates the temperature-dependent behavior of low energy phonons in YBa₂Cu₃O₆.55, revealing anisotropic charge density wave fluctuations and their coupling to phonons, supporting the unconventional nature of the CDW in this high-temperature superconductor.

Contribution

It provides the first detailed analysis of in-plane isotropy of phonon anomalies in YBa₂Cu₃O₆+x, linking phonon behavior to charge density wave fluctuations with combined experimental and theoretical evidence.

Findings

Anomalies linked to CDW fluctuations observed in both reciprocal planes.

Phonon anomalies match the symmetry of charge modulation.

Evidence of direct phonon-CDW coupling.

Abstract

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 qualitatively similar to those previously observed in the $(0, K, L)$ plane. This confirms the unconventional nature of this bi-dimensional CDW, which is not soft-phonon driven. With the support of first principles calculations, the symmetry of the anomalous phonon is identified and is found to match that of the charge modulation. This suggests in turn that these anomalies originate from a direct coupling between the phonons and the collective CDW excitations.