Landau theory of charge and spin ordering in the nickelates

TL;DR

This paper develops a Landau theory to explain charge and spin ordering in nickelates, linking these phenomena to structural distortions and electronic properties, and demonstrating its adaptability to different geometries.

Contribution

It introduces a phenomenological Landau theory that accounts for charge and spin orderings in nickelates, emphasizing the role of orthorhombic symmetry and nearly-nested spin density waves.

Findings

Charge ordering depends on orthorhombic distortion.

Nearly-nested spin density wave explains magnetic order.

First order phase transitions are characterized.

Abstract

Guided by experiment and band structure, we introduce and study a phenomenological Landau theory for the unusual charge and spin ordering associated with the Mott transition in the perovskite nickelates, with chemical formula RNiO$_3$, where R=Pr, Nd, Sm, Eu, Ho, Y, and Lu. While the Landau theory has general applicability, we show that for the most conducting materials, R=Pr, Nd, both types of order can be understood in terms of a nearly-nested spin density wave. Furthermore, we argue that in this regime, the charge ordering is reliant upon the orthorhombic symmetry of the sample, and therefore proportional to the magnitude of the orthorhombic distortion. The first order nature of the phase transitions is also explained. We briefly show by example how the theory is readily adapted to modified geometries such as nickelate films.