Phonon anomalies and lattice dynamics in superconducting oxychlorides Ca$_{2-x}$CuO$_2$Cl

TL;DR

This study investigates phonon behavior in superconducting Ca$_{2-x}$CuO$_2$Cl, revealing anomalies linked to charge modulations and highlighting discrepancies between experimental results and theoretical calculations.

Contribution

It provides the first detailed phonon dispersion analysis in oxychloride cuprates and compares these with other hole-doped cuprates, emphasizing the role of screening effects.

Findings

Anomalous dip in Cu-O bond stretching mode observed.

Maximum broadening of phonon mode at charge modulation vector.

Screening effects may cause overestimation of doping impacts in calculations.

Abstract

We present a comprehensive study of the phonon dispersion in an underdoped, superconducting Ca$_{2-x}$CuO$_2$Cl$_2$ crystal. We interpret the results using lattice dynamical calculations based on a shell model, and we compare the results, to other hole-doped cuprates, in particular to the ones isomorphic to La$_{2-x}$Sr$_x$CuO$_4$ (LSCO). We found that an anomalous dip in the Cu-O bond stretching dispersion develops in oxychlorides with a simultaneous marked broadening of the mode. The broadening is maximum at $\approx (\pi / (2a) ~ 0 ~ 0)$ that corresponds to the charge-modulations propagation vector. Our analysis also suggests that screening effects in calculations may cause an apparent cosine-shaped bending of the Cu-O bond-stretching dispersion along both the ($q$ 0 0) and ($q$ $q$ 0) directions, that is not observed on the data close to optimal doping. This observation suggests that the discrepancy between experimental data and \textit{ab-initio} calculations on this mode originates from an overestimation of the doping effects on the mode.