Possible flat-band ferromagnetism in an organic polymer

Abstract

Motivated from the flat-band form magnetism conceived in the physics of correlated electrons; we predict that a polymer of five-membered rings can exhibit band (i.e. itinerant) ferromagnetism. We have first identified that the right material having a flat one-electron dispersion should be polyaminotriazole, for which we have performed a generalized gradient approximation (GGA) calculation based on the spin density functional. The result shows that the ground state is indeed ferromagnetic when the flat band is made half-filled. From the structural optimization, we also show that the magnetism overcomes the Peierls instability, unlike conjugated polymers such as polyacetylene. We have also confirmed that the mechanism for the ferromagnetism is the flat-band ferromagnetism by comparing the first-principles wavefunction with those in the Hubbard model. Possibility of the bulk ferromagnetism for the three dimensional polyaminotriazole crystal with chemical carrier doping is also discussed.