Orbital ordering in frustrated Jahn-Teller systems
Abstract
We consider the superexchange in `frustrated' Jahn-Teller systems, such as the transition metal oxides NaNiO_2, LiNiO_2, and ZnMn_2O_4, in which transition metal ions with doubly degenerate orbitals form a triangular or pyrochlore lattice and are connected by the 90-degree metal-oxygen-metal bonds. We show that this interaction is much different from a more familiar exchange in systems with the 180-degree bonds, e.g. perovskites. In contrast to the strong interplay between the orbital and spin degrees of freedom in perovskites, in the 90-degree exchange systems spins and orbitals are decoupled: the spin exchange is much weaker than the orbital one and it is ferromagnetic for all orbital states. Due to frustration, the mean-field orbital ground state is strongly degenerate. Quantum orbital fluctuations select particular ferro-orbital states, such as the one observed in NaNiO_2. We also discuss why LiNiO_2 may still behave as an orbital liquid.
Cite
@article{arxiv.cond-mat/0201420,
title = {Orbital ordering in frustrated Jahn-Teller systems},
author = {M. V. Mostovoy and D. I. Khomskii},
journal= {arXiv preprint arXiv:cond-mat/0201420},
year = {2009}
}
Comments
5 pages, 3 figures