Title: Origins of bond and spin order in rare-earth nickelate bulk and heterostructures,
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.