Yu-Shiba-Rusinov-assisted Andreev transport in a molecular junction between superconductors

Magnetic impurities in superconductors host Yu-Shiba-Rusinov (YSR) bound states, whereas weak superconducting junctions sustain subharmonic charge transport through multiple Andreev reflections (MAR). We explore these phenomena using a molecular-scale junction created by placing a nickelocene (Nc) molecule atop an Fe adatom on Pb(111) and approaching it with a superconducting scanning-tunneling-microscope tip. Gradually increasing the junction transparency drives the system from conventional YSR spectroscopy in the tunneling regime to resonant MAR transport and, ultimately, to a Josephson-coupled state. The resulting spectra exhibit shifted MAR thresholds, asymmetric line shapes, and odd-even features that identify YSR-assisted MAR. A combined theoretical treatment of junction transparency and exchange coupling accounts for these observations, while density-functional calculations show that the Nc-Fe complex forms an antiferromagnetic ground state with a residual Fe spin- $\frac{1}{2}$ generating the YSR resonances. Our results demonstrate that the parameters governing YSR and MAR processes can be continuously tuned within a single superconducting nanojunction.