Flat-band ferromagnetism proposed for an organic polymer crystal

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Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five membered rings (polyaminotriazole) by Arita et al., [Phys. Rev. Lett. 88, 127202 (2002)], we have studied whether the magnetism can indeed occur as a bulk, i.e., in a three-dimensional crystal of the polymer, by means of the spin density functional calculation. We find that the intra-chain ferromagnetism is robust against crystallization as far as the flat band is made half-filled. We have further investigated the actual crystal doped with various compounds, where HF₂ is shown to put the system close to the bulk ferromagnetism while stronger anions such as BF₄ or PF₆ should be promising.