Flat-band ferromagnetism in organic polymers designed by a computer simulation

By coupling a first-principles, spin-density functional calculation with an exact diagonalization study of the Hubbard model, we have searched over various functional groups for the best case for the flat-band ferromagnetism proposed by R. Arita et al. [Phys. Rev. Lett. 88, 127202 (2002)] in organic polymers of five-membered rings. The original proposal (polyaminotriazole) has turned out to be the best case among the materials examined, where the reason why this is so is identified here. We have also found that the ferromagnetism in the polymer, originally proposed for the half-filled flat band, is stable even when the band filling is varied away from the half filling. All these make the ferromagnetism proposed here more experimentally inviting.