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

The tunnel junction between superconductors is the heart of many modern quantum information devices, such as superconducting qubits realized from quantum resonators with Josephson junctions. However, several tunneling mechanisms occur simultaneously in the superconducting tunnel junction, and thermal broadening further mixes them to obscure the identification of the tunneling mechanism. Here, we present a method to identify distinct tunneling modes in a tunable superconducting tunnel junction composed of a superconducting tip and sample in a scanning tunneling microscope (STM) [1]. As a testbed of the method, we implemented Pb coated tip to STM and made a tunnel junction with Pb single crystal. Combining the measurement of the relative decay constant of the tunneling current extracted from I-V-z spectroscopy, we identified the crossover of tunneling modes between single quasiparticle tunneling, multiple Andreev reflection, and Josephson tunneling with respect to the bias voltage. The measurement was complemented by the statistical analysis over the atomic disorder in the sample surface, where the distribution of relative decay constant displayed the transition of tunneling mechanism without any presumed physical model. Our 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 is crucial for quantum device application of superconductors.