Ferromagnetism in multi--band Hubbard models: From weak to strong Coulomb repulsion

TL;DR

This paper introduces a new mechanism for ferromagnetism in multi-band Hubbard models with triangles, demonstrating ferromagnetic ground states across various coupling regimes through analytical and numerical methods.

Contribution

It presents a novel ferromagnetism mechanism in Hubbard models with triangles, derived from an effective Hamiltonian in the strong coupling limit, applicable at different fillings.

Findings

Ferromagnetism occurs in quarter-filled, insulating cases over a wide parameter range.

The ferromagnetic ground state persists from flat-band to strong coupling limits.

Results also hold in metallic, non-quarter-filled cases.

Abstract

We propose a new mechanism which can lead to ferromagnetism in Hubbard models containing triangles with different on-site energies. It is based on an effective Hamiltonian that we derive in the strong coupling limit. Considering a one-dimensional realization of the model, we show that in the quarter-filled, insulating case the ground-state is actually ferromagnetic in a very large parameter range going from Tasaki's flat-band limit to the strong coupling limit of the effective Hamiltonian. This result has been obtained using a variety of analytical and numerical techniques. Finally, the same results are shown to apply away from quarter-filling, in the metallic case.