Flat-band ferromagnetism induced by off-site repulsions
  Density matrix renormalization group method is used to analyze how the nearest-neighbor repulsion V added to the Hubbard model on 1D triangular lattice and a railway trestle (t-t') model will affect the electron-correlation dominated ferromagnetism arising from the interference (frustration). Obtained phase diagram shows that there is a region in smaller-t' side where the critical on-site repulsion above which the system becomes ferromagnetic is reduced when the off-site repulsion is introduced. 
Ryotaro Arita, Yukihiro Shimoi, Kazuhiko Kuroki and Hideo Aoki