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

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.

6 pages, 4 figures, Supplemental Material