Ferromagnetism in the Hubbard model with orbital degeneracy in infinite dimensions

TL;DR

This paper investigates ferromagnetism in the Hubbard model with orbital degeneracy in infinite dimensions, revealing how doping influences magnetic phases and highlighting the role of the double-exchange mechanism in stabilizing metallic ferromagnetism.

Contribution

It demonstrates the persistence of ferromagnetism upon doping and clarifies the mechanisms stabilizing magnetic phases in an infinite-dimensional Hubbard model with orbital degeneracy.

Findings

Ferromagnetic phase with orbital order exists at 1/4-filling.

Doping leads to metallic ferromagnetism while antiferromagnetic phase disappears.

Double-exchange mechanism stabilizes metallic ferromagnetism in the model.

Abstract

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 $U^\prime$, the exchange $J$ (Hund coupling) and the pair hopping $J^\prime$, 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. At the 1/4-filling, we found two insulating phases; one is a ferromagnetic phase with alternating orbital order and the other is antiferromagnetic one 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 the double-exchange mechanism is relevant to stabilize metallic ferromagnetism in infinite dimensions.