Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygen-doped ${\mathrm{La}}_{1.94}{\mathrm{Sr}}_{0.06}{\mathrm{CuO}}_{4.035}$ $\left(T_c=38\text{K}\right)$ and ${\mathrm{La}}_2{\mathrm{CuO}}_{4+\delta }$ $\left(T_c=43\text{K}\right)$. Similar to results from optimally doped ${\mathrm{La}}_{1.85}{\mathrm{Sr}}_{0.15}{\mathrm{CuO}}_4$ $\left(T_c=35\text{K}\right)$, 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 ubiquitous in optimally doped superconductors.

• Received 25 October 2019
• Accepted 11 March 2020

DOI:https://doi.org/10.1103/PhysRevB.101.100504