Nonperturbative Ferromagnetism in the Hubbard Model for Quarter-Filling with One Extra Electron

Abstract

We present a Hubbard model with a special hopping matrix, which has highly degenerate single-electron energy eigenstatesbut has different properties than the flat-band Hubbard model.Our model has a paramagnetic ground state when the electron filling factor is less than or equal to quarter-filling.In the case of quarter-filling with one extra electron, the present model exhibits saturated ferromagnetism in the ground states for a sufficientlylarge value of on-site Coulomb interaction but does not for a small one.