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 TN2 in YVO3. Next we address the
variation of orbital and magnetic transition temperature, TOO and
TN1, 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.Comment: 25 pages, 10 figure