Universal proximity effects in hybrid superconductor-linker molecule-nanoparticle systems: The effect of molecular chirality

The superconductor proximity effect in systems comprising metallic nanoparticles (NPs) and molecules (NP/molecule/superconductor heterostructure) is an intriguing phenomenon that gives rise to important questions, from both fundamental and applicative perspectives, about the nature of Andreev reflections in nanoscale NPs and molecules and the interplay between the different energy scales, which remain hotly debated. In recent studies of such systems, a unique proximity effect was observed, manifested by an enhancement, rather than reduction, of the superconductor critical temperature, T_C, which can be related to higher order Andreev reflections of Cooper pairs that couple through the organic molecule linkers. In the present study, we investigate the proximity effect in such hybrid systems, using two types of superconducting films (Nb_0.17Re_0.83 and Nb), coupled Au or Ag NPs, via chiral or non-chiral molecule linkers. Non-chiral linkers lead to an enhancement of T_C after NP attachment, in agreement with previous results, while chiral linkers cause a decrease in T_C following NP adsorption. The results with chiral linkers can be explained by the following possible factors: the magnetic-like behavior that chiral molecules exhibit owing to their spin-filtering properties, which should enhance pair breaking, and strong spin-orbit coupling at the linker/NP interface that affects Andreev reflections between the superconductor and the NP and acts to reduce T_C within a two-band model we discuss. The insight gained from this work into the interaction between chiral molecules and superconductors is of importance for applications in chiral-based superconducting spintronics.