Frustrated magnetic interactions in a cyclacene crystal

TL;DR

This study explores how a crystal of cyclacenes exhibits magnetic properties due to unpaired electrons, with DFT calculations revealing complex antiferromagnetic interactions and frustrated spin phases in a two-dimensional carbon-based system.

Contribution

It demonstrates the emergence of frustrated magnetism in a cyclacene crystal and analyzes how molecular orientation affects magnetic interactions using DFT.

Findings

Cyclacene molecules host unpaired electrons leading to magnetic moments.

Intermolecular exchange interactions are antiferromagnetic and frustrated.

The crystal forms a bilayer of coupled antiferromagnetic triangular lattices.

Abstract

We study the emergence of magnetism and its interplay with structural properties in a two dimensional molecular crystal of cyclacenes, using density functional theory (DFT). Isolated cyclacenes with an even number of fused benzenes host two unpaired electrons in two topological protected zero modes, at the top and bottom carbon rings that form the molecule. We show that, in the gas phase, electron repulsion promotes an open-shell singlet with strong intramolecular antiferromagnetic exchange. We consider a closed packing triangular lattice crystal phase and we find a strong dependence of the band structure and magnetic interactions on the rotation angle of the cyclacenes with respect to the crystal lattice vectors. The orientational ground state maximizes the intermolecular hybridization, yet local moments survive. Intermolecular exchange is computed to be antiferromagnetic, and DFT predicts a broken symmetry $120^\circ$ spin phase reflecting the frustration of the intermolecular spin coupling. Thus, the cyclacene crystal realizes a bilayer of two antiferromagnetically coupled S = 1/2 triangular lattices. Our results provide a bottom-up route towards carbon based strongly correlated platforms in two dimensions.