PHASE-DIAGRAM OF A ONE-DIMENSIONAL DOUBLY-DEGENERATE HUBBARD-MODEL BYEXACT DIAGONALIZATION

Phase diagrams of a small-sized system of the one-dimensional doubly-d egenerate Hubbard model for the one-quarter-filled case are obtained f or various Coulomb interactions and hopping integrals by exact diagona lization. It is explicitly shown that a maximum spin-multiplicity grou nd state occurs when the Coulomb repulsions and the exchange interacti on are large compared with the bandwidth. It is found that mixed hoppi ng between orbitals has a little effect on the ferromagnetic correlati ons of the degenerate Hubbard model. The qualitative features of the p hase diagrams can be understood by physical insights based on strong c oupling estimates. It is also illustrated that the ferromagnetic prope rty is greatly enhanced when the system contains a localized orbital s tate.