Electron correlations and bond-length fluctuations in copper oxides: from Zhang--Rice singlets to correlation bags

TL;DR

This study uses first-principles multiconfiguration calculations to explore electron distributions and lattice couplings in copper oxides, supporting models of charge segregation and vibronic mechanisms related to superconductivity.

Contribution

It reveals near-degeneracy of Zhang--Rice and alternative states, and highlights strong electron-lattice coupling effects relevant to superconductivity models.

Findings

Zhang--Rice state nearly degenerate with Cu d9–O p5–Cu d9 state

Strong electron-lattice coupling causes large charge transfer effects

Results support charge segregation and vibronic models for superconductivity

Abstract

We perform first principles, multiconfiguration calculations on clusters including several CuO$_6$ octahedra and study the ground-state electron distribution and electron--lattice couplings when holes are added to the undoped $d^9 p^6$ configuration. We find that the so-called Zhang--Rice state on a single CuO$_4$ plaquette is nearly degenerate with a state whose leading configuration is of the form Cu $d^9$-- O $p^5$-- Cu $d^9$. A strong coupling between the electronic and nuclear motion gives rise to large inter-site charge transfer effects for half-breathing displacements of the oxygen ions. Under the assumption of charge segregation into alternating hole-free and hole-rich stripes of Goodenough \cite{jbg_02,jbg_03}, our results seem to support the vibronic mechanism and the traveling charge-density wave model from Refs.\cite{jbg_02,jbg_03} for the superconductivity in copper oxides.