Ferromagnetic Ground State of Two-Band Hubbard Model with One Electron per Site

We study the multiband mechanism to favour a ferromagnetic insulating ground state within a two-band Hubbard model. Besides perturbation theory we use exact diagonalization studies to examine the ground state of chains and 2D-lattices. According to second order perturbation theory the exact diagonalization yields a fully polarized ferromagnetic ground state, if the hopping between two ground state orbitals of neighbouring atoms t$\text{}_{gg}$ is small and the hopping between a ground state orbital and an excited orbital t$\text{}_{ge}$ dominates. However, in contrast to the suggestion from the second order perturbation theory this ferromagnetic state is stable only for very small hopping integrals U≫ t$\text{}_{ge}$>t$\text{}_{gg}$. For larger t $\text{}_{ge}$ quantum interference effects lead to complex magnetic structures.