In 2024, researchers at the Swiss Federal Laboratories for Materials Science and Technology (Empa) and their partners succeeded for the first time in exactly recreating a one-dimensional alternating Heisenberg model in a synthetic material, as reported by Empa. This theoretical quantum physics model, which has been known for almost 100 years, describes a linear concatenation of spins - a kind of quantum magnetism. The researchers led by Roman Fasel, head of Empa's nanotech@surfaces laboratory, have now been able to reconstruct the "sister model" in the laboratory. Researchers from the Max Planck Institute of Microstructure Physics in Halle (MPI), the International Iberian Nanotechnology Laboratory (INL) and the Technical University of Dresden were also involved, as reported by the MPI.
Second model with completely different properties
While the spins were alternately strongly and weakly linked in the alternating model, they are evenly linked in the new model, as Empa explains further in its press release. This seemingly small difference leads to fundamentally different properties: The spins of the homogeneous chain are strongly entangled and long-range correlated, and there is no energy gap between the ground state and the excited states. The alternating chain, on the other hand, develops an energy gap and its spins preferentially form strong pair bonds, with the correlations decreasing exponentially. The researchers were able to confirm these predictions of theoretical quantum physics exactly in their nanographene spin chains.
Both models were realized with nanographenes. These are tiny pieces of the two-dimensional carbon material graphene. By precisely controlling the shape of these pieces, the researchers can control their (quantum) physical properties. The goal is a material platform - a kind of "quantum Lego" - with which various quantum models and effects can be investigated experimentally.
The two Heisenberg experiments illustrate this: for the alternating spin chain model, the researchers used so-called "Clar's goblets" as the starting material. These are hourglass-shaped nanographene molecules consisting of eleven carbon rings. They used a different nanographene for the homogeneous Heisenberg chain: Olympicene, which consists of five rings and owes its name to its resemblance to the Olympic rings.
Practical applications with spin chains also possible
"We have now shown for the second time that theoretical models of quantum physics can be realized with nanographenes and that their predictions can therefore be tested experimentally," says Roman Fasel. Next, the researchers want to use their nanographenes to produce and investigate ferrimagnetic spin chains; although the magnetic moments in these align antiparallel, they do not cancel each other out completely. Of great interest are also two-dimensional spin lattices, which exhibit a much greater variety of phases than spin chains, including topological states, quantum spin liquids and exotic critical phenomena. This makes them particularly interesting, both for basic research and for practical applications.
After all, recreating models from quantum physics textbooks also has a practical purpose. Quantum technologies promise breakthroughs in communication, computing power, measurement technology and much more. However, quantum states are fragile and their effects are difficult to grasp. Research into real applications is correspondingly challenging. With the "quantum lego" made of nanographs, researchers hope to gain a better understanding of quantum effects and thus pave the way for usable quantum technologies.