INFLUENCE OF QUASI-PARTICLE DAMPING ON MAGNETIC STABILITY

We propose a modified alloy analogy for the single-band Hubbard model, by which we investigate the possibility of spontaneous ferromagnetism in narrow energy bands. It is shown that a proper definition of the f ictitious alloy enables self-consistent magnetic solutions to be found . The existence of spontaneous magnetism is mainly influenced by the l attice structure, the effective Coulomb coupling, and the band occupat ion. In accordance with the simple Stoner criterion, ferromagnetism ap pears in strongly correlated electron systems for band occupations, wh ich locate the chemical potential mu in regions of high quasiparticle density of states. Rather realistic Curie temperatures are found. The macroscopic magnetic properties explain themselves via temperature-dep endent quasiparticle densities of states, quasiparticle band structure s, and respective spectral densities. It is shown how quasiparticle da mping may depress quite substantially the stability of magnetic states by broadening corresponding spectral density peaks. Correlation effec ts lead to the expected splitting into two quasiparticle subbands (''H ubbard bands''), and under certain conditions to an additional exchang e splitting of each of these quasiparticle subbands, as well as to a s pin-dependent band narrowing, the combination of which gives rise to a n unconventional ''inverse'' exchange shift at certain positions of th e Brillouin zone.