Strongly entangled Quantum Spin Rings driven by H\"uckel rule

TL;DR

This paper explores how strong interactions in quantum spin rings, governed by Hückel aromaticity, lead to unconventional magnetic orders and degenerate ground states, advancing the design of quantum materials.

Contribution

It introduces a novel design principle linking Hückel aromaticity to quantum spin macrocycles with unique magnetic properties.

Findings

Strong interactions cause non-trivial antiferromagnetic order.

Electronic structure correlates with Hückel aromaticity.

Experimental realization via on-surface synthesis of carbon macrocycles.

Abstract

Quantum spin rings represent an intriguing platform for studying unconventional magnetic order and exotic quantum phases, and they are also promising materials for emerging quantum technologies. Conventional spin systems consist of a set of weakly interacting localized spins that are well described by the Heisenberg spin models. Here, we demonstrate that strong interactions between radical centers in macrocycles of different sizes lead to fluctuations in the total number of unpaired electrons and to non-trivial antiferromagnetic order that extends beyond the Heisenberg picture. We demonstrate that the electronic structure of these spin rings is governed by the concept of 4n/4n+2 H\"uckel (anti)aromaticity for even-membered rings, whereas odd-membered rings possess a highly degenerate frustrated magnetic ground state. The strongly coupled spin rings are experimentally realized through the on-surface synthesis of {\pi}-magnetic carbon-based macrocycles, which consist of [2]triangulene units. The close correlation between the electronic structure and the H\"uckel aromaticity rule is revealed by scanning tunneling spectroscopy and multireference calculations. This work establishes a novel design principle employing the concept of H\"uckel aromaticity for quantum spin macrocycles.