Ferromagnetism in the Hubbard model with orbital degeneracy in infinite dimensions

We study the ferromagnetism due to orbital degeneracy in the Hubbard model in infinite dimensions. The model contains the intraorbital repulsion U, the interorbital repulsion U^', the exchange J (Hund coupling) and the pair hopping J^', where all of them originate from the on-site Coulomb interaction. The ground state of the effective one-site problem was obtained by exact diagonalizations of finite clusters. At the 1/4 filling, we found two insulating phases; one is a ferromagnetic phase with alternating orbital order and the other is antiferromagnetic with uniform orbital order. If electrons are doped into the 1/4 filling, the ferromagnetic phase still survives and becomes metallic, while the antiferromagnetic phase disappears. This result indicates that a double-exchange mechanism is relevant to stabilize metallic ferromagnetism in infinite dimensions.