Oxygen Phonon Branches in YBa2Cu3O7

TL;DR

This study uses inelastic neutron scattering to analyze oxygen phonon dispersions in optimally doped YBa2Cu3O7, revealing strong electron-phonon coupling and temperature-dependent phonon anomalies related to superconductivity.

Contribution

It provides detailed experimental data on in-plane oxygen bond-stretching phonons and interprets complex anticrossing effects using lattice-dynamical models, advancing understanding of phonon behavior in high-Tc cuprates.

Findings

Steep downward dispersion of oxygen bond-stretching phonons in both a and b directions.

Identification of a sharp local phonon frequency minimum at q=0.27 r.l.u. upon cooling.

Confirmation of strong electron-phonon coupling in YBa2Cu3O7.

Abstract

We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa2Cu3O6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying the in-plane modes involving out-of-phase motion for the two Cu-O layers within a unit cell as well as those with in-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axis-polarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to T = 10 K at the wave vector q = 0.27 r.l.u..