Quantum spin model made from nanographene molecules

In 2024, Empa researchers and their partners successfully for the first time realized a so-called one-dimensional alternating Heisenberg model with a synthetic material (link to article). This theoretical quantum-physical model, known for nearly a century, describes a linear chain of spins – a type of quantum magnetism. Now, the researchers led by Roman Fasel, head of Empa's nanotech@surfaces laboratory, have also been able to reconstruct the model's “sibling” in the lab.

In the alternating model, spins are linked in a pattern of alternating strong and weak couplings. In the new model, however, the spins are evenly connected. This seemingly minor difference results in fundamentally different properties: The spins in the homogeneous chain are strongly entangled and exhibit long-range correlations, with no energy gap between the ground state and the excited states. In contrast, the alternating chain develops an energy gap, and its spins tend to form strong pairwise bonds, with correlations rapidly decreasing (exponentially). The researchers were able to precisely confirm these predictions of theoretical quantum physics in their nanographene spin chains. The corresponding findings have just been published in the latest edition of Nature Materials.

Both models were realized with nanographenes, tiny fragments of the two-dimensional carbon material graphene. By precisely controlling the shape of these fragments, the researchers can manipulate their (quantum) physical properties. The goal is to develop a material platform – a kind of “quantum Lego” – that allows various quantum models and effects to be experimentally studied.

The two Heisenberg experiments highlight this approach. For the alternating spin chain model, the researchers used so-called Clar's goblets as their starting material, hourglass-shaped nanographene molecules consisting of eleven carbon rings. For the homogeneous Heisenberg chain, they used a different nanographene: Olympicene, which consists of five rings and owes its name to its resemblance to the Olympic rings.