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Defect states and spin-orbital physics in doped vanadates Y1-xCaxVO3

We present a model for typical charged defects in weakly doped Y1-xCaxVO3 perovskites and study how they influence the magnetic and orbital order. Starting from a multiband Hubbard model, we show that the charge carriers introduced by doping are bound to the Ca defects with large binding energy of about 1 eV at small doping, and give rise to the in-gap absorption band observed in the optical spectroscopy. The central position of a generic Ca defect with eight equidistant vanadium neighbors implies a partly filled defect band and permits activated transport due to Coulomb disorder. We explore the effect of bound charge carriers on the dynamics of the (yz,zx) orbital and spin degrees of freedom in the context of a spin-orbital t-J model. After deriving the superexchange interactions around the doped hole, we show that the transition from G-type to C-type antiferromagnetic (AF) order is triggered by the kinetic energy of doped holes via the double-exchange mechanism. The defect states lead to local modification of orbital correlations within ferromagnetic chains along the c axis; some of them contain hole defects, while the charge-orbital coupling suppresses locally (yz,zx) orbital fluctuations in the others. Thereby, Ca defects provide a physical mechanism for spin-orbital dimerization along the ferromagnetic bonds, suggesting that, in the C-AF phase of weakly doped Y1-xCaxVO3, dimerization increases with doping.