Vacuum tunneling of superconducting quasiparticles from atomically sharp scanning tunneling microscope tips

We report on the study of atomically sharp superconducting tips for scanning tunneling microscopy and spectroscopy. The results clearly show vacuum tunneling of superconducting quasiparticles from atomically sharp tips. Observed deviations of the energy gap of the superconducting tip from its bulk value are attributed to the proximity effect. We show that a combination of a superconducting tip and an atomic resolution scanning tunneling microscope provides a means of achieving very high resolution local spectroscopy. We also discuss how this combination paves the way for a number of important applications.