Orbital degeneracy as a source of frustration in LiNiO$_2$

TL;DR

This paper investigates how orbital degeneracy causes frustration in LiNiO$_2$, revealing complex singlet phases and proposing explanations for its lack of magnetic order, with implications for related compounds.

Contribution

It introduces a spin-orbital model on the triangular lattice showing multiple singlet phases, explaining LiNiO$_2$'s properties and predicting magnetic behavior in NaNiO$_2$.

Findings

Identification of several singlet phases near the SU(4) point.

A phase with many low-lying singlets linked to dimer coverings.

Prediction of magnetic properties in NaNiO$_2$ based on orbital phases.

Abstract

Motivated by the absence of cooperative Jahn-Teller effect and of magnetic ordering in LiNiO$_2$, a layered oxide with triangular planes, we study a general spin-orbital model on the triangular lattice. A mean-field approach reveals the presence of several singlet phases between the SU(4) symmetric point and a ferromagnetic phase, a conclusion supported by exact diagonalizations of finite clusters. We argue that one of the phases, characterized by a large number of low-lying singlets associated to dimer coverings of the triangular lattice, could explain the properties of LiNiO$_2$, while a ferro-orbital phase that lies nearby in parameter space leads to a new prediction for the magnetic properties of NaNiO$_2$.