# Origins of bond and spin order in rare-earth nickelate bulk and heterostructures

## TL;DR

This paper investigates the mechanisms behind bond and spin order in rare-earth nickelates, revealing charge fluctuations drive bond order in bulk and spin fluctuations dominate in heterostructures, with results matching experimental data.

## Contribution

It provides a detailed theoretical analysis of charge and spin responses in RNiO3, clarifying the origins of bond and magnetic order in bulk and heterostructures.

## Key findings

- Charge fluctuations drive bond order in bulk RNiO3.
- Magnetic order in bulk is due to localized moments, not spin fluctuations.
- In heterostructures, spin fluctuations dominate, leading to spin-density-wave states.

## Abstract

We analyze the charge- and spin response functions of rare-earth nickelates RNiO3 and their heterostructures using random-phase approximation in a two-band Hubbard model. The inter-orbital charge fluctuation is found to be the driving mechanism for the rock-salt type bond order in bulk RNiO3, and good agreement of the ordering temperature with experimental values is achieved for all RNiO3 using realistic crystal structures and interaction parameters. We further show that magnetic ordering in bulk is not driven by the spin fluctuation and should be instead explained as ordering of localized moments. This picture changes for low-dimensional heterostructures, where the charge fluctuation is suppressed and overtaken by the enhanced spin instability, which results in a spin-density-wave ground state observed in recent experiments. Predictions for spectroscopy allow for further experimental testing of our claims.