Low-q dispersion of transverse acoustic phonons in YBa$_2$Cu$_3$O$_{7-\delta}$

TL;DR

This study uses high-resolution inelastic X-ray scattering to investigate transverse acoustic phonons in YBa₂Cu₃O₇−δ, revealing linear dispersion and consistent sound velocity, which challenges theories linking bending modes to charge carrier interactions.

Contribution

It provides the first detailed measurement of phonon dispersion in YBa₂Cu₃O₇−δ, showing no softening and confirming the absence of strong bending mode coupling.

Findings

Phonon dispersion remains linear down to q=0.02.

Sound velocity matches ultrasound measurements.

No evidence of bending mode softening or coupling.

Abstract

High-resolution ($\Delta E$=1.3 meV) inelastic X-ray scattering (IXS) in optimally doped untwinned YBa$_2$Cu$_3$O$_{7-\delta}$ single crystals show that the dispersion of transverse acoustic phonons in the a-direction remains linear with no softening due to bending modes in the long wavelength limit, down to q=0.02. The sound velocity is found to be 2700 +/- 70 m s$^{-1}$, in excellent agreement with ultrasound measurements, and unchanged upon cooling from room temperature to 98 K. These results rule out a strong coupling between bending dynamics of the CuO$_2$ planes and charge carriers in cuprates.