# Phonon anomalies with doping in superconducting oxychlorides Ca2-xCuO2Cl2

## TL;DR

This study investigates how doping affects phonon behavior in superconducting oxychlorides, revealing anisotropic effects and discrepancies between theoretical predictions and experimental observations at different doping levels.

## Contribution

It provides the first detailed measurement of Cu-O bond-stretching phonons in Ca$_2$CuO$_2$Cl$_2$ and compares experimental results with density functional theory, highlighting doping-dependent anisotropic effects.

## Key findings

- Phonon dispersion shows anisotropic doping effects.
- Density functional theory overestimates doping effects at optimal doping.
- Experimental results differ from theoretical predictions at certain doping levels.

## 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.