Anomalous Dispersion of LO Phonons in Oxygen-Doped La$_{2-x}$Sr$_{x}$CuO$_{4+\delta}$

TL;DR

This study uses inelastic neutron scattering to reveal anomalous phonon dispersion in oxygen-doped La$_{2-x}$Sr$_{x}$CuO$_{4+ ext{delta}}$, suggesting a link to charge fluctuations and stripe phenomena in high-temperature superconductors.

Contribution

It provides new evidence of phonon anomalies in oxygen-doped cuprates, highlighting their possible connection to charge correlations independent of structural instabilities.

Findings

Observed phonon softening halfway through the Brillouin Zone.

Anomalies are similar across different doping types with similar $T_c$.

Phonon anomalies are linked to charge fluctuations, not dopant-specific structural effects.

Abstract

Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygen doped La$_{1.94}$Sr$_{0.06}$CuO$_{4.035}$ ($T_c = 38\,\text{K}$) and La$_2$CuO$_{4+\delta}$ ($T_c = 43\,\text{K}$). Similar to results from optimally doped La$_{1.85}$Sr$_{0.15}$CuO$_4$ ($T_c = 35\,\text{K}$), we observe anomalous features in the dispersion of this half-breathing mode in the form of a softening halfway through the Brillouin Zone. Considering the differences in electronic structure and local environment between the oxygen- and strontium-doped compounds with similar $T_\text{c}$, we rule out a connection between the phonon anomaly and structural instabilities related to the specific dopant type. We interpret the phonon anomaly as a signature of correlated charge fluctuations possibly connected to stripes.