Orbital Fluctuations in the RVO3 Perovskites

Abstract

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 V 3+ ions in the RVO 3 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 LaVO 3 . We argue that the GdFeO 3 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 YVO 3 . Next we address the variation of orbital and magnetic transition temperature, T OO and T N1 , in the RVO 3 perovskites, after extending the spin-orbital model by the crystal-field and the orbital interactions which arise from the GdFeO 3 and Jahn-Teller distortions of the VO 6 octahedra. We further find that the orthorhombic distortion which increases from LaVO 3 to LuVO 3 plays a crucial role by controlling the orbital fluctuations, and via the modified orbital correlations influences the onset of both magnetic and orbital order.