Scanning tunneling microscope studies of the superconductor proximity effect

Abstract

Scanning tunneling microscopy and spectroscopy are employed in studies of the proximity effect between normal metals and superconductors. The experimental configuration is unique, in that the tunneling current flows in parallel to the interfaces between different materials. The samples are superconducting wires consisting of ordered arrays of submicron diameter normal-metal filaments, either Cu or Ni (a ferromagnet), embedded in a NbTi superconducting matrix. By taking topographic images simultaneously with current-voltage curves, we map with nanometer resolution the local quasiparticle density of states. Two main issues are addressed in this work. The first is the spatial variation of the superconductor gap as a function of distance from the boundary between the normal-metal and the superconductor. We find that the healing length of the gap on the superconducting side is much larger than the superconductor coherence length. The second is the observation of pronounced Tomasch oscillations and bound states arising from multiple Andreev reflections of quasiparticles propagating in the plane perpendicular to the tunneling current.