LaVO_3 : A true Kugel-Khomskii system

We show that the t^2_2g perovskite LaVO_3, in its orthorhombic phase, is a rare case of a system hosting an orbital-ordering Kugel’-Khomskii phase transition rather than being controlled by the Coulomb-enhanced crystal-field splitting. We find that, as a consequence of this, the magnetic transition is close to (and even above) the superexchange driven orbital-ordering transition, whereas typically magnetism arises at much lower temperatures than orbital ordering. Our results support the experimental scenario of orbital ordering and G-type spin correlations just above the monoclinic-to-orthorhombic structural change. To explore the effects of crystal-field splitting and filling, we compare to YVO_3 and t^1_2g titanates. In all these materials the crystal field is sufficiently large to suppress the Kugel’-Khomskii phase transition