6 Correlation Energies in Distorted 3 d - t 2 g Perovskite Oxides

Using an effective low-energy Hamiltonian derived from the first-principles electronic structure calculations for the narrow t2g bands of YTiO3, LaTiO3, YVO3, and LaVO3, we evaluate the contributions of the correlation energy (EC) to the stability of different magnetic structures, which can be realized in these distorted perovskite oxides. We consider two approximations for EC , which are based on the regular perturbation theory expansion around a nondegenerate Hartree-Fock ground state. One is the second order of perturbation theory, which allows us to compare the effects of local and nonlocal correlations. Another one is the local t-matrix approach, which allows us to treat some higher-order contributions to EC . The correlation effects systematically improve the agreement with the experimental data and additionally stabilize the experimentally observed Gand C-type antiferromagnetic (AFM) structures in YVO3 and LaVO3, though the absolute magnitude of the stabilization energy is sensitive to the level of approximations and 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. Amongst 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.