Tuning effective pairing potential through atomic scale control of superconductor heterostructures

Previous experiments showed that superconductivity persists in ultrathin films of conventional superconductors, even in films that are only 1-2 atomic layers thick. However, it has also been implied that the interface and substrate can strongly influence the electronic properties in the quasi two-dimensional regime. In this work, we fabricated a heterostructure with a normal metal layer (Ag here) placed in between a superconducting film (Pb here) and an insulating substrate, and measured its superconductivity with scanning tunneling spectroscopy subsequently. The most striking observation is the requirement of an overlayer (L$_{Pb})$ to recover the superconductivity, while the recovery thickness is linearly dependent on the underlayer thickness. Considering the renormalization of pairing interaction strength for Cooper pair in the hybrid structure, the ``effective attractive potential'' model was developed in this work. It could serve as a model with predictive power to describe the general behavior of superconductor heterostructures. Moreover, the physical origin of some discrepancies in the reported transition temperature as a function of film thickness is partly elucidated by this work as well.