Phonon anomalies with doping in superconducting oxychlorides Ca2-xCuO2Cl2

Abstract

We measure the dispersion of the Cu-O bond-stretching phonon mode in the high-temperature superconducting parent compound Ca${}_2$CuO${}_2$Cl${}_2$. Our density functional theory calculations predict a cosine-shaped bending of the dispersion along both the ($\xi$00) and ($\xi \xi$0) directions, while comparison with previous results on Ca${}_{1.84}$CuO${}_2$Cl${}_2$ show it only along ($\xi$00), suggesting an anisotropic effect which is not reproduced in calculation at optimal doping. Comparison with isostructural La${}_{2-x}$Sr${}_x$CuO${}_4$ suggests that these calculations reproduce well the overdoped regime, however they overestimate the doping effect on the Cu-O bond-stretching mode at optimal doping.