Behavior of superconductivity in a Pb/Ag heterostructure

Abstract

The superconducting proximity effect is a long-standing topic of great importance in condensed matter physics. A crucial but unresolved issue is which interfacial and material details determine the efficiency of the proximity effect. In this paper, we study an epitaxially grown superconductor/normal metal (SC-NM) heterostructure (Pb/Ag) and find a spatially constant superconducting gap determined by local tunneling spectroscopy and magnetoresponse measurements, despite the highly mismatched Fermi surfaces between individual Pb and Ag epitaxial layers and the large differences in the lattice constants and electronic densities of states in the separate components. The uniform superconducting gap is in contrast to the spatially varying pair potential with a discontinuity at the interface theoretically predicted for an ideal SC-NM junction and experimentally observed previously in several lateral SC-NM junctions. We experimentally verify that the transmission of electrons across the interface in the vertical Pb/Ag heterostructure is high enough that a new band structure emerges even at the single-particle level. Our experimental results call for further theoretical work in order to develop predictive power for the proximity effect starting from realistic, materially relevant microscopic models.