$d-p$ model and spin-orbital order in the vanadium perovskites.

Using the multi-band $d-p$ model and unrestricted Hartree-Fock approximation we investigate the electronic structure and spin-orbital order in three-dimensional VO$_3$ lattice. The main aim of this investigation is testing if simple $d-p$ model, with partly filled $3d$ orbitals (at vanadium ions) and $2p$ orbitals (at oxygen ions), is capable of reproducing correctly nontrivial coexisting spin-orbital order observed in the vanadium perovskites. We point out that the multi-band $d-p$ model has to include partly filled $e_g$ orbitals at vanadium ions. The results suggest weak self-doping as an important correction beyond the ionic model and reproduce the possible ground states with broken spin-orbital symmetry on vanadium ions: either $C$-type alternating orbital order accompanied by $G$-type antiferromagnetic spin order, or $G$-type alternating orbital order accompanied by $C$-type antiferromagnetic spin order. Both states are experimentally observed and compete with each other in YVO$_3$ while only the latter was observed in LaVO$_3$. Orbital order is induced and stabilized by particular patterns of oxygen distortions arising from the Jahn-Teller effect. In contrast to time-consuming \textit{ab-initio} calculations, the computations using $d-p$ model are very quick and should be regarded as very useful in solid state physics, provided the parameters are selected carefully.