Motivated by the absence of cooperative Jahn-Teller effect and of magnetic ordering in LiNiO2, 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 LiNiO2, while a ferro-orbital phase that lies nearby in parameter space leads to a new prediction for the magnetic properties of NaNiO2.
@article{arxiv.cond-mat/0401122,
title = {Orbital degeneracy as a source of frustration in LiNiO$_2$},
author = {F. Vernay and K. Penc and P. Fazekas and F. Mila},
journal= {arXiv preprint arXiv:cond-mat/0401122},
year = {2009}
}