Existence of a metallic ferromagnetic phase in models for undoped manganites

The existence of a metal-insulator transition in the ferromagnetic state of models for undoped manganites is discussed here, using numerical techniques applied to the $e_g$-orbital degenerate Hubbard model tightly coupled with Jahn-Teller distortions. The ground-state phase diagram is presented in the plane defined by the electron-phonon coupling $\lambda$ and Coulomb interaction u. In contrast to the standard one-band Hubbard model for cuprates, the metallic phase is found to exist for finite values of both $\lambda$ and u in the present $e_g$-orbital Hubbard model even at half-filling, due to the Fermi-surface topology which is incompatible with the staggered orbital ordering concomitant to the insulating phase. Based on the present results, a possible scenario for the colossal magnetoresistive effect is discussed in undoped manganites.