Above-gap differential conductance dips in superconducting point contacts

Abstract

Point-contact Andreev spectroscopy is one of the most powerful techniques for obtaining quantitative energy and momentum-resolved information on the electronic properties of a superconductor. However, appropriate measurements often reveal unexpected conductance features that cannot be adequately modeled using conventional theory. In particular, these include conductance dips at voltage biases slightly exceeding the expected energy gap value for a superconductor. Guided by recent point-contact measurements of proximity-induced superconductivity in topological materials, we explain these features by inhomogeneous superconducting state in the studied material, conditionally divided into two parts with different gap values and a semitransparent potential barrier between them. To test this assumption, we designed a model experiment that mimics the main features of the model. The heterostructure under study is formed by an almost ideal point contact, formed using a new technology of self-textured nanofilaments based on the resistive-switching effect in a TiO2 film, a Pt counter-electrode, and a proximized Al/Ni0.5Cu0.5/NbN system, where superconducting correlations from the NbN superconductor are penetrating into a weak ferromagnetic Ni0.5Cu0.5 alloy in contact with an Al film. For a quantitative description of the conductance dips observed in all such devices and their evolution with temperature, we propose an extension of the conventional theory, the main element of which is a non-equilibrium-driven superconducting-to-normal transition and a crossover from coherent to sequential transmission in the Al/NiCu bilayer. Understanding the origin of the conductance dips imposes some restrictions on the explanation of induced superconductivity and paves the way for discovering novel superconducting materials with advanced properties.