INL researchers publish in Nature Materials on building atomic-scale ‘spin chains’ to study quantum excitations

March 14, 2025

In a recent publication in Nature Materials, INL researchers João Henriques and Joaquín Fernández-Rossier, through an international collaborative effort, explored a very sought-after quantum spin system.

In a breakthrough that brings the strange world of quantum spins into sharper focus, researchers have successfully created and studied tiny chains of molecular rings, unlocking new insights into how quantum spin excitations behave in these atomic-scale systems.

This work centres on a famous quantum physics model known as the spin-1/2 Heisenberg chain, a system where individual spins (a fundamental quantum property related to magnetism) interact in a one-dimensional line. Theorists predicted decades ago that these chains should have “gapless” excitations, meaning tiny disturbances in the spin system can occur with extremely small energy costs as the chain gets longer. However, building such chains at the nanoscale and measuring these effects directly has proven extremely challenging – until now.

The research team achieved this by linking special carbon-based molecules called olympicenes – shaped like miniature Olympic rings – into precise linear chains. Each olympicene hosts an unpaired electron, creating a perfect space for quantum spin physics. Using a powerful technique called inelastic electron tunnelling spectroscopy, the scientists were able to directly measure how the system’s energy spectrum evolved with chain length.

The results confirmed the long-predicted trend. As the chains grew longer, the energy needed to excite the spins steadily decreased, following a power-law decay. In a chain of 50 olympicenes, the team saw a nearly V-shaped spectrum, directly revealing the expected gapless behaviour in the “thermodynamic limit” (essentially what happens if the chain were infinitely long).

One particularly exciting twist came from chains with an odd number of spins. In these systems, the team could actually image a single spinon – an exotic quantum excitation that emerges when the chain’s spins collectively rearrange. Spinons have been predicted and detected indirectly in past experiments, but directly visualising one as a kind of “standing wave” is a major step forward in understanding these excitations.

This work not only confirms a foundational theory of quantum magnetism but also opens a path towards engineering and studying quantum spin systems at the atomic scale. In the future, such tailored spin chains could be useful for developing new materials with exotic magnetic properties or for quantum computing.

This research involved collaboration with INL – International Iberian Nanotechnology Laboratory, Empa, Technical University of Dresden, Max Planck Institute of Microstructure Physics, Universidade de Santiago de Compostela, Universidad de Alicante, and University of Bern.