Low-temperature characterization of Nb-Cu-Nb weak links with Ar ion-cleaned interfaces

TL;DR

This paper presents low-temperature characterization of Nb-Cu-Nb superconductor-normal metal-superconductor weak links with Ar ion cleaning, demonstrating their potential for sensitive superconducting sensors and comparing their interface quality to Al-Cu counterparts.

Contribution

It introduces a two-step fabrication process for Nb-Cu-Nb SNS devices with high-quality interfaces and demonstrates their application in hybrid magnetic flux sensors.

Findings

Interface quality comparable to Al-Cu weak links.

Successful integration with aluminum tunnel junctions.

Potential for enhanced sensitivity in superconducting sensors.

Abstract

We characterize niobium-based lateral Superconductor (S) - Normal metal (N) - Superconductor weak links through low-temperature switching current measurements and tunnel spectroscopy. We fabricate the SNS devices in two separate lithography and deposition steps, combined with strong argon ion cleaning before the normal metal deposition in the last step. Our SNS weak link consists of high-quality sputtered Nb electrodes that are contacted with evaporated Cu. The two-step fabrication flow enables great flexibility in the choice of materials and pattern design. A comparison of the temperature-dependent equilibrium critical supercurrent with theoretical predictions indicates that the quality of the Nb-Cu interface is similar to that of evaporated Al-Cu weak links. Aiming at increased sensitivity, range of operation temperatures, and thermal isolation, we investigate how these SNS structures can be combined with shadow-evaporated aluminum tunnel junctions for sensor applications that utilize the superconducting proximity effect. To this end, we demonstrate a hybrid magnetic flux sensor based on a Nb-Cu-Nb SNS junction, where the phase-dependent normal metal density of states is probed with an Al tunnel junction.