Orbital Fluctuations in the RVO3 Perovskites

The properties of Mott insulators with orbital degrees of freedom are described by spin-orbital superexchange models, which provide a theoretical framework for understanding their magnetic and optical properties. We introduce such a model derived for (xy)(1)(yz/zx)(1) configuration of V3+ ions in the RVO3 perovskites, R = Lu,Yb,...,La, and demonstrate that {yz,zx} orbital fluctuations along the c axis are responsible for the huge magnetic and optical anisotropies observed in the almost perfectly cubic compound LaVO3. We argue that the GdFeO3 distortion and the large difference in entropy of C-AF and G-AF phases is responsible for the second magnetic transition observed at T-N2 in YVO3. Next we address the variation of orbital and magnetic transition temperature, Too and T-N1, in the RVO3 perovskites, after extending the spin-orbital model by the crystal-field and the orbital interactions which arise from the GdFeO3 and Jahn-Teller distortions of the VO6 octahedra. We further find that the orthorhombic distortion which increases from LaVO3 to LuVO3 plays a crucial role by controlling the orbital fluctuations, and via the modified orbital correlations influences the onset of both magnetic and orbital order.