Peculiar near-contact regime of Andreev reflection at the breakdown of a tunnel junction

Recently we introduced non-contact Andreev reflection (NCAR) - a new experimental approach to quantify Andreev reflection in a tunable tunnel junction [1]. NCAR detects Andreev current in STM geometry through renormalized tunnneling decay rate. This technique therefore enables atomic-scale imaging of Andreev current and quantitative detection of the superconducting state, while avoiding invasive and mesoscale mechanical contacts of the well-established PCAR methodology. One peculiar observation enabled by NCAR is that Andreev reflection does not have a monotonous dependence on tip-sample separation. Here we will explore this effect using both classical and unconventional superconductors as model systems. We will describe the fundamental connection between the tunneling barrier dependence of Andreev reflection and the order in perturbation theory responsible for the tunneling current. The combination of non-contact and near-contact regimes of Andreev reflection then enables a definitive probe of unconventional superconductivity, which we will demonstrate using the canonical FeSe. Moreover, the quantitative comparison to tight-binding modeling enabled by this technique provides even deeper insight, including possible inference of the order parameter symmetry. 1. W. Ko, J. Lado, P. Maksymovych, Nano Lett. 22 (2022) 4042.