The researchers in the Quantum Detection Microscopy group at CIC nanoGUNE, in collaboration with chemists at CiQUS of the University of Santiago de Compostela, have demonstrated that adding or removing a single molecule from a quantum chain can fundamentally change its interaction with a superconductor. The key lies in a property as simple as parity, that is, whether the chain contains an even or odd number of units. While chains with an odd number interact magnetically with the superconductor, those with an even number do not. The results of this study have been published in the prestigious journal Nature.

The chains are formed by triangulenes, small triangular fragments of graphene that host a spin (S=1). When these magnetic units couple with other, their spins interact and form a collective quantum state known as the Haldane phase, which was theoretically predicted more than four decades ago. One of their most unique properties is that a chain created with integer spin units exhibits fractional spin states at both ends (S=1/2).

The research team proposed to study what happens when these fractional states come into contact with a superconductor. This, however, presented an experimental challenge. Gold surfaces provide the necessary platform for manufacturing carbon nanostructures using on-surface synthesis techniques, but this metal does not have superconductive properties. Other materials, such niobium, are superconductors, but do not offer the same chemical environment for carrying out this type of synthesis.

To overcome this limitation, the research team developed a strategy based on growing a gold film with a thickness of 25 to 30 atomic layers on a niobium crystal. The gold film acquires superconductivity due to its proximity to the superconductor. The result is a platform that combines the characteristics of both materials: it maintains the chemical environment of gold and exhibits, from an electronic perspective, the properties of a superconductor.

The measurements taken using tunnelling microscopy and spectroscopy showed a clear parity effect. In the chains formed by an odd number of triangulenes, the research team observed a characteristic sign of the interaction between a magnetic moment and the superconducting state. In contrast, in the chains with an even number of triangulenes, these signs disappear, demonstrating the decisive role of the number of units in the interaction with the superconductor.

“Magnetic molecules are candidates for use as quantum bits, and a superconductor offers an environment in which to read them and protect them from noise. The challenge is to control their interaction. These results demonstrate a method for turning the interaction of the chains with the superconductor on or off by changing their length by a single unit”, explains Nacho Pascual, Ikerbasque research professor at CIC nanoGUNE. “Beyond the core result, the study establishes a platform for combining two tools that have been difficult to bring together up to now: the creation of atomically precise molecular materials and superconductivity”, he adds. “This will enable the controlled exploration of how magnetic states, quantum correlations and topological phases interact with a superconducting condensate, by designing these interactions molecule by molecule”, explains Nacho Pascual.

This study was funded by MOLSENSE, a coordinated project by the Spanish Research Agency (AEI), and CONSPIRA, an Advanced Grant project from the European Research Council (ERC), led by Nacho Pascual at CIC nanoGUNE. Both projects complement each other by combining the chemical synthesis of custom-made molecules with their study at the atomic scale using tunnelling microscopy, to produce graphene nanostructures and control their quantum state with microwave signals, with a view to developing future sensors and quantum technologies.