Mechanism and Control Parameters of the Coupled Structural and Metal-Insulator Transition in Nickelates

TL;DR

This paper develops a theoretical framework for understanding the coupled electronic and structural phase transition in rare-earth nickelates, emphasizing the role of electronic disproportionation and lattice stiffness in driving the metal-insulator transition.

Contribution

It introduces a Landau theory with two coupled order parameters to explain the transition, highlighting the key control parameters involved.

Findings

Transition driven by proximity to electronic disproportionation instability

Coupling between electronic and lattice modes is crucial

Identifies susceptibility and lattice stiffness as key control parameters

Abstract

Rare-earth nickelates exhibit a remarkable metal-insulator transition accompanied by a structural transition associated with a lattice `breathing' mode. Using model considerations and first-principles calculations, we present a theory of this phase transition, which reveals the key role of the coupling between the electronic and lattice instabilities. We show that the transition is driven by the proximity to an electronic disproportionation instability which couples to the breathing mode, thus cooperatively driving the system into the insulating state. This allows us to identify two key control parameters of the transition: the susceptibility to electronic disproportionation and the stiffness of the lattice mode. We show that our findings can be rationalized in terms of a Landau theory involving two coupled order parameters, with general implications for transition-metal oxides.