Statistical detection of Josephson, Andreev, and single quasiparticle currents in scanning tunneling microscopy

We present a method to identify distinct tunneling modes in tunable superconducting tunnel junction composed of superconducting tip and sample in scanning tunneling microscope. Combining the measurement of the relative decay constant of tunneling current extracted from I-V-z spectroscopy with its statistical analysis over the atomic disorders in the sample surface, we identified the crossover of tunneling modes between single quasiparticle tunneling, multiple Andreev reflection, and Josephson tunneling with respect to the bias voltage. The method enables one to determine the particular tunneling regime independently of the spectral shapes, and to reveal the intrinsic modulation of Andreev reflection and Josephson current that would be crucial for quantum device application of superconductors.