Interplay of charge and spin correlations in nickel perovskites

TL;DR

This paper develops a generalized t-J Hamiltonian for nickelates, incorporating charge, spin, and electron-phonon interactions, to explain complex charge and spin phenomena observed experimentally.

Contribution

It introduces a comprehensive model Hamiltonian for nickelates that includes carrier-spin, spin-spin, and electron-phonon couplings, addressing phenomena not captured by previous models.

Findings

Holes are nearly localized due to magnetic confinement effects.

Electron-phonon coupling leads to polaron formation in nickelates.

The model qualitatively explains experimental charge and spin ordering phenomena.

Abstract

Analyzing the motion of low--spin $(s=1/2)$ holes in a high--spin $(S=1)$ background, we derive a sort of generalized t--J Hamiltonian for the $\rm NiO_2$ planes of Sr--doped nickelates. In addition to the rather complex carrier--spin and spin--spin couplings we take into account the coupling of the doped holes to in--plane oxygen breathing modes by a Holstein--type interaction term. Because of strong magnetic confinement effects the holes are nearly entirely prelocalized and the electron--phonon coupling becomes much more effective in forming polarons than in the isostructural cuprates. In the light of recent experiments on $\rm La_{2-x}Sr_xNiO_4$ we discuss how the variety of the observed transport and charge/spin--ordering phenomena can be qualitatively understood in terms of our model Hamiltonian.