Flat-band ferromagnetism proposed for an organic polymer crystal

TL;DR

This study explores the potential for flat-band ferromagnetism in a three-dimensional organic polymer crystal, demonstrating that intra-chain ferromagnetism persists upon crystallization and identifying dopants that promote bulk ferromagnetism.

Contribution

It provides the first computational evidence that flat-band ferromagnetism can be realized in a bulk organic polymer crystal, extending previous single-chain theories to three-dimensional structures.

Findings

Intra-chain ferromagnetism is stable in the crystal when the flat band is half-filled.

Doping with HF$_2$ enhances proximity to bulk ferromagnetism.

Stronger anions like BF$_4$ or PF$_6$ are promising for ferromagnetic behavior.

Abstract

Motivated from the flat-band ferromagnetism conceived theoretically for a single chain of five membered rings (polyaminotriazole) by Arita {\it et al.}, [Phys. Rev. Lett. {\bf 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 HF$_2$ is shown to put the system close to the bulk ferromagnetism while stronger anions such as BF$_4$ or PF$_6$ should be promising.