Origin of metal-insulator transition in rare-earth Nickelates

TL;DR

This paper investigates the origin of the metal-insulator transition in rare-earth nickelates, revealing that magnetic interactions primarily drive the transition, with structural distortions enabling spin state changes that open an electronic gap.

Contribution

It demonstrates that magnetic interactions are the main cause of the MIT, with structural distortions facilitating spin state disproportionation in Ni ions.

Findings

Magnetic interactions primarily drive the MIT.

Structural distortions enable spin state disproportionation.

Disproportionation into high-spin and low-spin states opens an electronic gap.

Abstract

Rare-earth nickelates RNiO3 (R=rare-earth element) exhibit three kinds of phase transitions with decreasing temperature: a structural transition from a pseudo-cubic to a monoclinic phase, a metal- insulator transition (MIT), and a magnetic transition from a paramagnetic state to an ordered one. The first two occur at the same temperature, which has led to a consensus that the MIT is driven by lattice distortions. We show here that the primary driving force for the MIT is magnetic; however because of the unusual d7 configuration of Ni, additional flexibility in spin configurations are also needed which symmetry-lowing structural deformations make possible. The latter enable Ni to disproportionate into two kinds: a high-spin and a low-spin configuration, which allow the system to reduce its unfavorable orbital moment and also open a gap.