title: Orbital ordering in frustrated Jahn-Teller systems with 90 degrees exchange.
abstract: We show that superexchange interactions in frustrated Jahn-Teller systems with transition metal ions connected by the 90 degrees metal-oxygen-metal bonds (e.g., NaNiO2, LiNiO2, and ZnMn2O4) are much different from those in materials with the 180 degrees bonds. In the 90 degrees -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.
authors.affiliation: Materials Science Center Rijksuniversiteit Groningen, Nijenborgh 4, The Netherlands.
authors.forename: M V
authors.initials: MV
authors.lastname: Mostovoy
authors.forename: D I
authors.initials: DI
authors.lastname: Khomskii