Parity-dependent coupling of molecular spin chains to a superconductor

Collective behaviour in quantum materials can produce excitations carrying only a fraction of the quantum numbers of their constituents. A notable case occurs in chains of spin-1 units¹, in which a topological gap protects the interior of the chain, whereas spin-1/2 states, half the spin of the units that form it, emerge at the ends². In short chains, these fractional edge spins couple to each other and their topological protection makes them attractive for hybrid spin-superconductor devices. Whether such fractionalization survives the proximity of a superconducting condensate, however, remains an open question. Here we grow molecular Haldane chains of antiferromagnetically coupled spin-1 triangulene units on a proximitized Au(111)/Nb(110) surface and resolve a parity-dependent coupling of their spin-1/2 edge states to the superconductor by scanning tunnelling spectroscopy. Odd-length chains host Yu-Shiba-Rusinov (YSR) bound states inside the superconducting gap, originating from a global S = 1 ground state, whereas even-length chains form an S = 0 ground state decoupled from the superconductor. A two-site model attributes this alternation to the sign and strength of inter-edge interactions, independently confirmed by extra-gap spin excitations in tunnelling spectra. Many-body spin excitations of the S = 1 chain are also detected, decoupled from the superconductor by the much larger Haldane gap. The length-tunable coupling of the edge spins to superconductors opens a route towards molecular spin qubits based on π-conjugated carbon architectures.