Researchers from the Quantum Sensing Microscopy group at CIC nanoGUNE, in collaboration with chemists from CiQUS at the University of Santiago de Compostela, have demonstrated that adding or removing a single molecule from a quantum chain can radically 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 of units interact magnetically with the superconductor, those with an even number do not. The findings of this study have been published in the prestigious journal Nature.
The chains consist of triangulenes, small triangular fragments of graphene that carry a spin (S=1). When these magnetic units are linked together, their spins interact and form a collective quantum state known as the Haldane phase, theoretically predicted more than four decades ago. One of its most remarkable properties is that a chain built from integer-spin units develops fractional spin states at both ends, namely quantum objects with half that spin (S=1/2).
The research team set out to investigate what happens when these fractional states come into contact with a superconductor. Addressing this question, however, posed an experimental challenge. Gold surfaces provide the necessary platform for fabricating carbon nanostructures with atomic precision using on-surface synthesis techniques, but gold itself is not superconducting. Other materials, such as niobium, are superconductors, although they do not provide the same chemical environment required for this type of synthesis.
To overcome this limitation, the researchers developed a strategy based on growing a gold film between 25 and 30 atomic layers thick on a niobium crystal. The gold film becomes superconducting through its proximity to the underlying superconductor. The result is a platform that combines the characteristics of both materials: it preserves the chemical environment of gold while exhibiting the electronic properties of a superconductor.
Measurements performed using scanning tunnelling microscopy and spectroscopy revealed a clear parity effect. In chains containing an odd number of triangulenes, the researchers observed a characteristic signature of the interaction between a magnetic moment and the superconducting state. By contrast, these signatures disappeared in chains with an even number of triangulenes, demonstrating the decisive role played by the number of units in determining the interaction with the superconductor.
“Magnetic molecules are promising candidates for quantum bits, or qubits, and a superconductor provides an environment in which they can be read out and protected from noise. The challenge is to control their interaction. These results demonstrate a method for switching the interaction between the chains and the superconductor on or off by changing their length by just a single unit,” explains Nacho Pascual, Ikerbasque Professor at CIC nanoGUNE.
“Beyond the fundamental finding, this work establishes a platform that brings together two tools that have until now been difficult to combine: the construction of molecular materials with atomic precision and superconductivity,” he adds. “This will make it possible to explore, in a controlled way, how magnetic states, quantum correlations and topological phases interact with a superconducting condensate, designing these interactions molecule by molecule,” says Nacho Pascual.
This research was funded by MOLSENSE, a coordinated project supported by the Spanish State Research Agency, and CONSPIRA, a European Research Council (ERC) Advanced Grant project led by Nacho Pascual at CIC nanoGUNE. The two projects complement each other by combining the chemical synthesis of tailor-made molecules with their study at the atomic scale using scanning tunnelling microscopy, with the aim of fabricating graphene nanostructures and controlling their quantum states using microwaves, paving the way for future quantum sensors and technologies.
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