Physical origin of the buckling in CuO$_2$: Electron-phonon coupling and Raman spectra

TL;DR

This paper explains the buckling of CuO$_2$ planes in cuprates through an electric field caused by valence differences, linking it to electron-phonon coupling and Raman spectra, with experimental validation across different compounds.

Contribution

It provides a theoretical model connecting electric fields, electron-phonon coupling, and structural buckling in cuprates, supported by experimental data.

Findings

Electric field across CuO$_2$ planes explains buckling.

Fano line shape reveals strong electron-phonon coupling.

Buckling amplitude matches structural data.

Abstract

It is shown theoretically that the buckling of the CuO$_{2}$ planes in certain cuprate systems can be explained in terms of an electric field across the planes which originates from different valences of atoms above and below the plane. This field results also in a strong coupling of the Raman-active out-of-phase vibration of the oxygen atoms ($B_{1g}$ mode) to the electronic charge transfer between the two oxygens in the CuO$_{2}$ plane. Consequently, the electric field can be deduced from the Fano-type line shape of the $B_{1g}$ phonon. Using the electric field estimated from the electron-phonon coupling the amplitude of the buckling is calculated and found to be in good agreement with the structural data. Direct experimental support for the idea proposed is obtained in studies of YBa$_{2}$Cu$_{3}$O$_{6+x}$ and Bi$_{2}$Sr$_{2}$(Ca$_{1-x}$Y$_{x}$)Cu$_{2}$O$_{8}$ with different oxygen and yttrium doping, respectively, including antiferromagnetic samples. In the latter compound, symmetry breaking by replacing Ca partially by Y leads to an enhancement of the electron-phonon coupling by an order of magnitude.