Ferromagnetism in two band metals: The very strong coupling limit

The main purpose of this paper is to examine the occurrence of ferromagnetism in metals at the very strong coupling regime (U→∞) in the single-site approximation (SSA). The metal is described by a realistic two hybridized bands (V), one including Hubbard correlation and the other an uncorrelated one. We parametrize the ratio of the band widths and their centers as well. In this method the Hamiltonian is approximated by an effective and simpler one in which only one site retains the full correlation (U) while in the others acts an internal field, the self-energy. This field does not depend on the wave vector and is self-consistently determined by imposing the translation invariance of the problem, a coherent potential approximation (CPA) procedure. We also introduce Roth's wave vector dependent band shift Wk,σ. For several total electronic occupation numbers (ntot) we compare the spin dependent free energies with the corresponding paramagnetic ones at T=0. We conclude making a critical analysis of Roth's method for this problem arguing that the existence of a stable ferromagnetic state in this regime depends on a strong competition among Wk,σ, the limit U→∞ and V.