New spin states could build molecular quantum bits on surfaces

Katerina Vaxevani and colleagues at CIC nanoGUNE-BRTA have demonstrated parity-dependent coupling between molecular spin chains and a superconductor. The researchers grew molecular Haldane chains of antiferromagnetically coupled spin-1 triangulene units on a surface and used scanning tunnelling spectroscopy to observe how the chains interact with a superconducting condensate. Odd-length chains hosted Yu-Shiba-Rusinov bound states, while even-length chains remained decoupled, revealing a mechanism driven by the interaction between edge states. This length-tunable coupling, the work shows, opens a route toward molecular spin qubits based on carbon architectures. The creation of these molecular spin chains relies on a proximitized Au(111)/Nb(110) surface, a platform that allows for on-surface synthesis at elevated temperatures while preserving superconducting correlations. Stepwise annealing of molecular precursors induces Ullmann coupling and subsequent cyclodehydrogenation, resulting in triangulene chains of varying lengths across the gold surface. The reconstruction of the gold surface and the superconducting gap…