Ordering Phenomena in Transition Metal Oxides

Magnetic and optical properties of cubic vanadates   

The superexchange interactions in cubic vanadates are highly frustrated; they involve both spin and orbital degrees of freedom. When xy orbitals are filled by one electron, the spin-orbital superexchange model for cubic vanadates [1] explains that ferromagnetic interactions are enhanced in one of the cubic directions due to quantum fluctuations, leading to the C-type antiferromagnetic (AF) phase observed in LaVO₃. We use exact diagonalization combined with mean-field theory to investigate the phase diagram of this model at T=0 [2]. The spin-orbit coupling competes with Hund's exchange and triggers a novel phase, with the ordering of t_2g orbital magnetic moments stabilized by the tilting of VO₆ octahedra. It explains qualitatively spin canting and reduction of magnetization observed in YVO₃. We show that the Jahn-Teller effect stabilizes the orbital order and the G-type AF phase in YVO₃ at low temperatures, while at higher temperatures C-AF phase is favored instead. In this latter phase one finds modulation of magnetic exchange constants even in absence of lattice distortions. Finally, partial sum rules for the optical multiplet transitions will be introduced on the example of LaVO₃ [3]. They allow to analyze separately the temperature and polarization dependence of different components of the optical multiplet, determined by by the underlying spin and orbital correlations which follow from the spin-orbital model. Thereby the optical data provides deep insight into the complex spin-orbital physics and the role played by orbital fluctuations.

[1] G. Khaliullin, P. Horsch, and A. M. Oles, Phys. Rev. Lett. 86, 3879 (2001).
[2] P. Horsch, G. Khaliullin, and A. M. Oles, Phys. Rev. Lett. 91, 257203 (2003).
[3] G. Khaliullin, P. Horsch, and A. M. Oles, cond-mat/0403459, unpublished (2004).