Orbital ordering in frustrated Jahn-Teller systems with 90° exchange -: art. no. 227203

We show that superexchange interactions in frustrated Jahn-Teller systems with transition metal ions connected by the 90degrees metal-oxygen-metal bonds (e.g., NaNiO2, LiNiO2, and ZnMn2O4) are much different from those in materials with the 180degrees bonds. In the 90degrees-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. Though the mean-field orbital ground state is strongly degenerate, quantum orbital fluctuations select particular ferro-orbital states. We explain the orbital and magnetic ordering observed in NaNiO2 and show that LiNiO2 is not a spin-orbital liquid.