Defect-Induced Orbital Polarization and Collapse of Orbital Order in Doped Vanadium Perovskites

We explore mechanisms of orbital-order decay in the doped Mott insulators R_{1-x}(Sr,Ca)_{x}VO_{3} (R=Pr,Y,La) caused by charged (Sr,Ca) defects. Our unrestricted Hartree-Fock analysis focuses on the combined effect of random charged impurities and associated doped holes up to x=0.5. The study is based on a generalized multiband Hubbard model for the relevant vanadium t_{2g} electrons and includes the long-range (i) Coulomb potentials of defects and (ii) electron-electron interactions. We show that the rotation of t_{2g} orbitals, induced by the electric field of defects, is a very efficient perturbation that largely controls the suppression of orbital order in these compounds. We investigate the inverse participation number spectra and find that electron states remain localized on few sites even in the regime where orbital order is collapsed. From the change of kinetic and superexchange energy, we can conclude that the motion of doped holes, which is the dominant effect for the reduction of magnetic order in high-T_{c} compounds, is of secondary importance here.