Masters Defense: Bilal Kousar

Proximity Effect in InAs/MoRe Nanowire Hybrids

The study of proximity effect in semiconductor/superconductor nanowire hybrids is valuable both from a fundamental and practical point of view. Superconductors are known for being perfect conductors and showcase ideal diamagnetism. On the other hand, most of modern electronics, and indeed device physics, is built around the ability to tune electron density in semiconductor materials using external, capacitively coupled electric fields. A superconductor in good electrical contact with a semiconductor induces superconducting features through a process known as the ‘proximity effect’. A combination of two material classes therefore offers a versatile platform for new device physics and gate tunable superconductivity, as well as potential new applications including Majorana bound states, gatemons, Andreev qubits.

Most of the experimental works involving hybrid devices use aluminum as the superconductor, which has one of the smallest Bc and Tc of all known superconductors. This serves as a motivation for utilizing superconductors with potentially better properties than Al, such as higher transition temperatures, critical fields and superconducting energy gaps. This thesis investigates fundamental characteristics of proximity effect in hybrid devices based on InAs nanowires and molybdenum-rhenium alloy (MoRe) with superconducting gap ∆ and Tc up to an order of magnitude larger than Al.

We consider the proximity effect in InAs/MoRe hybrids under controllable parameters such as gate voltage, temperature and magnetic field. In addition to characterizing bulk MoRe film, device geometries including NS and SNS are investigated. Tunneling spectroscopy of the NS device yields an estimated induced gap of at least 0.9 meV with a high subgap bound state density. Turning to the Josephson junction devices, we find a maximum critical current Ic of 23 nA, which can be pinched off at lower gate voltages. Voltage bias spectroscopy of the SNS device in the tunneling regime reveals a gate dependent subgap structure, arising from the multiple Andreev reflections (MAR) in the presence of a single resonant level. In the open regime, a continuous supercurrent branch as well as finite bias MAR resonances are observed. A superconducting gap of 1.10 meV is extracted from the MAR peak positions. In conclusion, a large induced gap combined with the high density of bound states in the NS devices and a gate tunable supercurrent with well resolved MAR resonances in the SNS devices make MoRe a useful material to incorporate into semiconductor/superconductor hybrids with epitaxial/pristine interfaces.