FERROMAGNETISM IN MULTIBAND HUBBARD MODELS - FROM WEAK TO STRONG COULOMB REPULSION

We propose a mechanism which can lead to ferromagnetism in Hubbard mod els containing triangles with different on-site energies. It is based on an effective Hamiltonian that we derive in the strong coupling limi t. Considering a one-dimensional realization of the model, we show tha t 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 numeri cal techniques. Finally. the same results are shown to apply away from quarter-filling, in the metallic case.