Inelastic neutron scattering has been used to study the in-plane Cu-O bond-stretching mode in oxygendoped La1.94Sr0.06CuO4.035 (T-c = 38 K) and La2CuO4+delta (T-c = 43 K). Similar to results from optimally doped La1.85Sr0.15CuO4 (T-c = 35 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-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.
Tejsner, T., Piovano, A., Tutueanu, A., Romer, A. T., Wells, B. O., Grivel, J-C., Boehm, M., & Udby, L. (2020). Anomalous dispersion of longitudinal optical phonons in oxygen-doped La2-xSrxCuO4+δ. Physical Review B, 101(10). https://doi.org/10.1103/physrevb.101.100504