Orbital degeneracy as a source of frustration inLiNiO2

Abstract

Motivated by the absence of cooperative Jahn-Teller effect and of magnetic ordering in ${\mathrm{LiNiO}}_{2}$, 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 $\mathrm{SU}(4)$ 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 ${\mathrm{LiNiO}}_{2}$, while a ferro-orbital phase that lies nearby in parameter space leads to a new prediction for the magnetic properties of ${\mathrm{NaNiO}}_{2}$.