Y. Motome et M. Imada, Effects of electron correlation, orbital degeneracy and Jahn-Teller coupling in perovskite manganites, J PHYS JPN, 68(1), 1999, pp. 16-19
Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling i
n double-exchange models are examined for perovskite Mn oxides. We study th
e 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 indica
ted by experimental results, and investigate their ground-state properties
by the quantum Monte Carlo method. At half filling where the number of e(q)
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, h
owever, substantially slower than the mean-field results if the Jahn-Teller
coupling is absent. The synergy between the strong correlation and the Jah
n-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 Coulom
b 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 mod
el, moreover exhibits a uniform ordering of d(x)2(-y)2 orbitals over a wide
region of doping in agreement with experimental indications.