Bond disproportionation and dynamical charge fluctuations in the perovskite rare earth nickelates

TL;DR

This paper develops a comprehensive theoretical model for rare earth nickelates, explaining their electronic properties, charge fluctuations, and spectral responses, and validates bond disproportionation as a key mechanism behind their phase transitions.

Contribution

It introduces a full orbital, full correlation double cluster approach that accurately models charge fluctuations and spectral responses, confirming bond disproportionation as the core mechanism.

Findings

Strong charge fluctuations consistent with bond disproportionation

Excellent agreement with experimental resonant x-ray spectra

Validation of bond disproportionation theory for nickelates

Abstract

We present a theory describing the local electronic properties of the perovskite rare earth nickelates--materials which have negative charge transfer energies, strong O $2p$ -- Ni $3d$ covalence, and breathing mode lattice distortions at the origin of highly studied metal-insulator and antiferromagnetic ordering transitions. Utilizing a full orbital, full correlation double cluster approach, we find strong charge fluctuations in agreement with a bond disproportionation interpretation. The unique double cluster formulation permits the inclusion of necessary orbital degeneracies and Coulomb interactions to calculate resonant x-ray spectral responses, with which we find excellent agreement with well-established experimental results. This previously absent, crucial link between theory and experiment provides validation of the recently proposed bond disproportionation theory, and provides an analysis methodology for spectroscopic studies of engineered phases of nickelates and other high valence transition metal compounds.