Doped spin–orbital Mott insulators: Orbital dilution versus spin–orbital polarons

Here we compare two kinds of doping in transition metal oxides by considering defects due to the substitution of the transition metal or rare earth element. The focus is on insulating t2g orbital systems and we present various mechanisms that lead to reorganization of spin–orbital order. Substitutional doping of d4-transition metal by d3 ions generally results in removal of the local orbital degrees of freedom and the exchange on hybrid d4−d3 bonds modifying locally (or globally) spin–orbital order. Different routes of rearrangement of spin–orbital patterns are also found when considering doping by d2 ions. Here we demonstrate that in the regime of strong impurity-host coupling ferro-orbital type ordering is favored. This analysis can find application to ruthenate oxides with Mn or Cr replacing the Ru atoms. In contrast, charged (Sr,Ca) defect replacing R ion in the R1−x(Ca,Sr)xVO3 (R=La,Y) perovskites generates a spin–orbital polaron in the defect cube. We show that t2g orbital rotations are then induced near the charged defect and a doped hole disturbs orbital order. As a result, the collapse of G-type orbital order occurs but C-AF spin order stays robust under increasing doping