Magnetic frustration and fractionalization in oligo(indenoindenes)

TL;DR

This paper investigates the magnetic interactions and ground states of oligo(indenoindenes), revealing frustrated spin chains, antiferromagnetic order, and topologically nontrivial valence-bond solid states with fractional edge spins.

Contribution

It classifies the magnetic behaviors of different oligo(indenoindenes) isomers and identifies a topologically nontrivial valence-bond solid ground state similar to the AKLT model.

Findings

Frustrated S=1/2 Heisenberg chain behavior in some isomers

Antiferromagnetic order in other isomers

Valence-bond solid ground state with fractional edge spins

Abstract

Poly(indenoindenes) are {\pi}-conjugated ladder carbon polymers with alternating hexagons and pentagons hosting one unpaired electron for each five-membered ring in the open-shell limit. Here we study the main magnetic interactions that are present in finite oligo(indenoindenes) (OInIn), classifying the six possible isomers in two different classes of three isomers each. One class can be rationalized by frustrated S = 1/2 Heisenberg chains, with ferromagnetic interactions between neighbour sites and antiferromagnetic interactions between the next neighbours. The other class is characterized by the more trivial antiferromagnetic order. Employing several levels of theory we further show that the ground state of one of the isomers is a valence-bond solid (VBS) of ferromagnetic dimers (S = 1). This is topologically similar to that of the Affleck-Kennedy-Lieb-Tasaki (AKLT) model, which is known to show fractional S = 1/2 states at the edges.