Mott transition and ordering in the nickelates

The nickelates, RNiO₃ (R is a rare earth atom), are one of the canonical families of pseudo-cubic perovskites undergoing a Mott metal-insulator transition. They have been studied in bulk for decades. In addition to abrupt changes in the resistivity, the materials show interesting ordering phenomena below the Metal-Insulator Transition (MIT). Both antiferromagnetism (with an unusual 4 spin periodicity) and charge ordering are observed, mainly through elastic neutron scattering.

Attention has returned to the area recently as the nickelates have been grown in "oxide heterostructures" (films, interfaces, and superlattices) in an attempt to control their properties, e.g. the MIT and, more (extremely!) optimistically, engineer high temperature superconductivity. There is surprisingly little theory on these materials. Basic questions seem still in debate. Should the nickel valence be considered 3+ or 4+, or an admixture, and is this critical to the physics? Why the period four magnetic order? How crucial is the coupling to phonons? People even disagree on whether the matierials should be called Mott, charge-transfer, or band insulators!

Given all this uncertainty of basic questions, prospects for really controlling the heterostructures seem less than bright. Probably a better theoretical understanding is needed to make modeling possible. Currently the majority of that effort is in ab initio methods such as LDA+U and LDA+DMFT to nickelate heterostructures.

^ TOP

Our work:

We've been trying simpler approaches in hopes that at least some aspects of the nickelates might be understood more intuitively. So far we have two theory papers on this topic. In the first, we just use phenomenological Landau theory, motivated by very simple band structure theory and measurements, to discuss the ordering in the insulating states. This Landau theory can just be solved by hand on paper. This is a lot less work than LDA+DMFT but of course it is also much less quantitative! Anyway, the Landau theory does a pretty nice job of explaining several experimental observations, and accounting for effects like the orthorhombic distortion in the nickelates.

Read the paper.

In our second paper, we considered more microscopic questions. The Landau theory led us to think that much of the ordering could be understood for a weak to intermediate coupling point of view, in which it is the states near the Fermi surface that are most important. In this case, a two band model which reproduces properly the Fermi surface ought to suffice. We studied a minimal such model in a simple Hartree-Fock approximation. Among other things, this model is easily extended from the 3d bulk to heterostructures. Full disclosure: it is controversial whether this minimal model can be sufficient to describe the nickelates.

Read the paper (especially the Discussion section) and decide for yourself!.