Correlation energies in distorted 3d-t2g perovskite oxides

Using an effective low-energy Hamiltonian derived from the first-principles electronic structure calculations for the narrow t _2g bands of YTiO₃, LaTiO₃, YVO₃, and LaVO₃, we evaluate the contributions of the correlation energy E _c to the stability of different magnetic structures that can be realized in these distorted perovskite oxides. We consider two approximations for E _c that are based on regular perturbation theory expansion around a nondegenerate Hartree-Fock ground state. One is the second order of perturbation theory, which allows comparing the effects of local and nonlocal correlations. The other is the local t-matrix approach, which allows treating some higher-order contributions to E _c . The correlation effects systematically improve the agreement with the experimental data and additionally stabilize the experimentally observed G- and C-type antifer-romagnetic (AFM) structures in YVO₃ and LaVO₃, although the absolute magnitude of the stabilization energy is sensitive to the level of approximations and is somewhat smaller in the t-matrix method. The nonlocal correlations additionally stabilize the ferromagnetic ground state in YTiO₃ and the C-type AFM ground state in LaVO₃. Among two inequivalent transition-metal sites in the monoclinic structure, the local correlations are stronger at the sites with the least distorted environment. Limitations of the regular perturbation-theory expansion for LaTiO₃ are also discussed. The text was submitted by the authors in English.