Sharp optical phonon softening close to optimal doping in La$_{2-x}$Ba$_x$CuO$_{4+\delta}$

TL;DR

This study observes a sharp phonon anomaly in La$_{2-x}$Ba$_x$CuO$_{4+ $ ext{δ}$ close to optimal doping, indicating strong electron-phonon interactions possibly linked to stripe order and superconductivity.

Contribution

First direct observation of a Kohn-like phonon anomaly at optimal doping in La$_{2-x}$Ba$_x$CuO$_{4+ ext{δ}}$, revealing detailed phonon behavior near superconductivity.

Findings

Detection of a sharp phonon anomaly at q=(0.3, 0, 0)

Anomaly persists near optimal doping in superconducting state

Possible mode splitting due to stripe order

Abstract

We report a direct observation of a sharp Kohn-like anomaly in the doubly degenerate copper-oxygen bond-stretching phonon mode occurring at $\mathbf{q}\mathrm{=(0.3, 0,0)}$ in La$_{2-x}$Ba$_x$CuO$_{4+\delta}$ with $\mathrm{x=0.14\pm0.01}$, thanks to the high $\mathbf{Q}$ resolution of inelastic x-ray scattering. This anomaly is clearly seen when the inelastic signal is analysed using a single mode but is also consistent with a two mode hypothesis possibly due to a splitting of the degenerate modes due to symmetry breaking stripes. Our observation shows that the effect persists at the stripe propagation vector in a superconducting system close to optimal doping.