Innovative use of nanographenes achieves breakthrough in quantum modelling

October 31, 2024

A recent breakthrough in quantum materials research, published in Nature Nanotechnology, introduces a powerful platform for exploring and controlling topological phases in quantum systems.

This study was performed by scientists at INL, Empa – Swiss Federal Laboratories for Materials Science and Technology, the Technical University of Dresden and Max Planck Institute of Microstructure Physics. INL researchers played a crucial role by providing the theoretical calculations essential to this advance.

The international team developed a unique system of nanographene-based chains, whose building blocks are known as “Clar’s goblets”. Using a technique called ‘on-surface synthesis’, the researchers created alternating-exchange Heisenberg spin chains, allowing for targeted spin manipulation within a controlled structure.

The Heisenberg model, a foundational concept in quantum mechanics, describes how spins (intrinsic angular momenta of particles like electrons) interact with one another. In this paper, the Heisenberg spin chains are specially constructed from linked Clar’s goblets, nanographenes where each part of the molecule (a “site” in the chain) hosts a spin.

By covalently linking the Clar’s goblets, the researchers could precisely control properties such as chain length and exchange interactions at the atomic level. Scanning tunnelling microscopy further allowed the team to investigate the magnetic properties of these chains, monitoring spin behaviours through the spectral response.

Joaquín Fernández-Rossier and João Henriques

João Henriques, a PhD candidate at INL and a key researcher in this study, explains “in this study, the theoretical team made three important contributions to understanding spin behaviour in specially designed nanographene chains. First, they developed models to interpret experimental scanning tunnelingl microscopy data, which confirmed the presence of unique edge spins in one type of chain, while showing their absence in another. Second, they identified key topological differences between the chains. Finally, the team explained how atomically precise measurements can reveal the momentum resolved energy dispersion of excitations in these nanographene structures”.

A key finding in this study was the observation of “triplons” (gapped magnetic excitations within the spin chains). The researchers mapped the dispersion of these triplons across the chains and observed that spin interactions within each chain vary depending on its structure and termination. This intricate control of spin properties highlights the platform’s potential to explore distinct topological quantum phases.

These findings have applications in quantum computing and quantum simulations, where understanding and manipulating topological phases could lead to transformative technologies. Future research will focus on enhancing the coherence of these spin systems by isolating them from metallic substrates, a step towards scalable quantum devices based on custom-designed carbon nanomaterials.

Using special nanographene molecules, the researchers were able to discern a theoretical model from quantum physics. Image: Empa

Joaquín Fernández-Rossier, group leader at INL, concludes “this work is yet another example of how nanographenes are great building blocks to build artificial spin lattices and scanning tunnelling microscopy is an awesome tool to measure their spin excitations and unveil the secrets of quantum magnetism”.

Text and Photography by Catarina Moura, Science Communication Officer