Probing unconventional superconductivity by tunneling Andreev reflection

Andreev reflection (AR) is an electronic transport process at the junction of a normal metal and a superconductor, where the electrons in the normal metal transform to the Cooper pairs by retroreflecting holes and conducts current across the junction. The process is highly sensitive to the superconducting order parameters and functions as a tool to directly probe the superconductivity. Based on AR, we developed a new technique, tunneling Andreev reflection (TAR), by applying AR to the tunnel junction in scanning tunneling microscope (STM) [1]. Specifically, we precisely tune the STM tip-sample distance to systematically study the AR as a function of the tunneling barrier height. Since the AR is a higher order tunneling process compared to the normal electron tunneling, the relative decay rate of the tunneling conductance increases inside the superconducting gap, whose specific shape depends on the nature of the superconductivity. We measured decay rate spectra on FeSe and compared with the theoretical calculations to identify unconventional superconductivity and d-wave pairing symmetry [2].