Probing Proximity-Induced Superconductivity in InAs Nanowires Using Built-In Barriers

Bound states in superconductor-nanowire hybrid devices play a central role, carrying information on ground-state properties (Shiba or Andreev states) or on the topological properties of the system (Majorana states). The spectroscopy of such bound states relies on the formation of well-defined tunnel barriers, usually defined by gate electrodes, which results in smooth tunnel barriers. Here we used thin InP segments embedded into InAs nanowire during the growth process to form a sharp built-in tunnel barrier. Gate dependence and thermal-activation measurements are used to confirm the presence and estimate the height of this barrier. By coupling these wires to superconducting electrodes we investigate the gate-voltage dependence of the induced gap in the nanowire segment, which we can understand using a simple model based on Andreev bound states. Our results show that these built-in barriers are promising as future spectroscopic tools.