FERROMAGNETISM IN THE HUBBARD-MODEL - INSTABILITY OF THE NAGAOKA STATE ON THE TRIANGULAR, HONEYCOMB AND KAGOME LATTICES

In order to analyse the lattice dependence of ferromagnetism in the tw o-dimensional Hubbard model we investigate the instability of the full y polarised ferromagnetic ground state (Nagaoka state) on the triangul ar, honeycomb and kagome lattices. We mainly focus on the local instab ility applying single spin flip variational wave functions which inclu de majority spin correlation effects. The question of global instabili ty and phase separation is addressed in the framework of Hartree-Fock theory. We find a strong tendency towards Nagaoka ferromagnetism on th e non-bipartite lattices (triangular, kagome) for more than half filli ng. For the triangular lattice we find the Nagaoka state to be unstabl e above a critical density of n = 1.887 at U = infinity, thereby signi ficantly improving former variational results. For the kagome lattice the region where ferromagnetism prevails in the phase diagram widely e xceeds the flat band regime. Our results even allow the stability of t he Nagaoka state in a small region below half filling. In the case of the bipartite honeycomb lattice several disconnected regions are left for a possible Nagaoka ground state.