  Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in
double-exchange models are examined for Mn perovskite oxides. We study the
undoped Mott 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 in the
experimental indications, and investigate their ground-state properties by
quantum Monte Carlo method. At half filling where the number of $e_{g}$
electron is one per site on average, the Coulomb interaction opens a Mott gap
and induces a staggered orbital ordering. The opening of the Mott 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 Mott gap amplitude and reproduces
realistic amplitudes and stabilization energy of the Jahn-Teller distortion.
Upon doping, the orbital ordering stabilized by the Coulomb interaction is
destroyed immediately. Toward the metal-insulator transition, the short-ranged
orbital correlation is critically enhanced in metals, which should be related
to strong incoherence of charge dynamics observed in experiments. Our model,
moreover, exhibits a uniform ordering of $d_{x^{2}-y^{2}}$ orbital in a wide
region of doping in agreement with experimental indications.
