Effects of electron correlation, orbital degeneracy and Jahn-Teller coupling in perovskite manganites

Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in double-exchange models are examined for perovskite Mn oxides. We study the undoped insulator as well as metal-insulator transitions by hole doping, and especially strong incoherence of ferromagnetic metal. We derive models where all the spins are fully polarized in two-dimensional planes as indicated by experimental results, and investigate their ground-state properties by the quantum Monte Carlo method. At half filling where the number of e_g electrons is one per site on average, the Coulomb interaction opens a Mott gap and induces a staggered orbital ordering. The opening of the gap is, however, substantially slower than the mean-field results if the Jahn-Teller coupling is absent. The synergy between the strong correlation and the Jahn-Teller coupling largely enhances the charge gap amplitude and reproduces realistic amplitudes and stabilization energy of the Jahn-Teller distortion. Doping of carriers destroys the orbital ordering stabilized by the Coulomb interaction. The short-ranged orbital correlation is critically enhanced in metals toward the metal-insulator transition, which should be related to the strong incoherence of charge dynamics; observed in experiments. Our model, moreover, exhibits a uniform ordering of d_x2-y2 orbitals over a wide region of doping in agreement with experimental indications.