Parity-dependent coupling of molecular spin chains to a superconductor

In the microscopic world of quantum physics, particles sometimes behave as if they are part of a larger, collective whole rather than independent individuals. When this happens in certain materials, the fundamental rules of the universe can appear to change, allowing particles to split into smaller, fractional pieces that carry only a fraction of their usual properties. This phenomenon, known as fractionalization, is a hallmark of exotic states of matter where strong interactions between particles create new, unexpected behaviors. One of the most fascinating examples occurs in one-dimensional chains of magnetic atoms. In these chains, the magnetic spins of the atoms align in a specific pattern that protects the ends of the chain, leaving them with a unique, fractional magnetic character that does not exist in the middle of the chain. Scientists have long wondered what would happen if they brought these fragile, fractional edge states into contact with a superconductor—a material that conducts electricity with zero resistance and expels magnetic fields. The question was whether the superconductor would destroy these delicate quantum states or if they could coexist, potentially forming the basis for new types of quantum computers.

A team of researchers has now answered this question by building tiny molecular chains on a special surface and watching how they interact with a superconductor. They created chains made of triangular carbon molecules, known as triangulene, which act like a string of magnetic beads. When these chains are short, the magnetic properties at the two ends of the chain interact with each other. If the chain has an odd number of these triangular units, the two ends behave like a single magnetic object with a net spin. If the chain has an even number of units, the two ends cancel each other out, resulting in a non-magnetic state. The researchers placed these chains on a thin film of gold that was sitting on top of a superconducting metal called niobium. This setup allowed the gold film to become superconducting itself, creating a "proximitized" environment where the superconducting properties seeped into the gold without destroying the delicate chemical structure of the carbon chains.

Using a highly sensitive microscope that can measure the flow of electricity at the atomic scale, the team observed a striking difference depending on the length of the chain. When they looked at chains with an odd number of triangular units, they found clear signs that the magnetic ends were talking to the superconductor. The microscope detected specific energy peaks inside the superconducting gap, which is the range of energy where electricity usually cannot flow. These peaks, known as Yu-Shiba-Rusinov states, are a signature that a magnetic moment is sitting right on the superconductor, creating a localized disturbance. This confirmed that the fractional spin at the end of the odd-length chain remained active and coupled to the superconducting electrons.

In contrast, the chains with an even number of units told a different story. Because the two ends of these chains cancelled each other out to form a non-magnetic state, they did not create any of the special energy peaks inside the superconducting gap. The superconductor remained undisturbed by the chain, effectively ignoring it. However, the researchers did see signs of interaction just outside the superconducting gap, where the energy required to flip the magnetic state of the chain was visible. This proved that the two ends were still interacting with each other, but their combined non-magnetic nature kept them decoupled from the superconductor itself.

The researchers also discovered that the strength of the interaction between the two ends of the chain changed as the chains got longer. In shorter chains, the ends were close enough to feel each other strongly, which determined whether the chain acted as a magnetic or non-magnetic object. As the chains grew longer, this interaction weakened, and the ends began to act more like independent particles. The team used a theoretical model to explain this behavior, showing that the alternating magnetic states of the chains were the key factor. They found that the magnetic ground state of the chain, whether it was a single magnetic unit or a cancelled-out pair, dictated how it would couple to the superconductor.

This work demonstrates that the topological protection of these fractional spin states is robust enough to survive in a superconducting environment. The ability to switch the magnetic behavior of the chain simply by adding or removing a single unit, and to read this state out through its interaction with a superconductor, offers a new way to control quantum information at the molecular scale. The study suggests that carbon-based molecular architectures could serve as building blocks for hybrid quantum devices, where the stability of topological states meets the power of superconductivity. By proving that these exotic quantum states can be engineered and controlled on a superconducting surface, the researchers have opened a path toward creating more stable and complex quantum systems for future technologies.