Novel Josephson junction geometries in NbCu bilayers fabricated by focused ion beam microscope

We explore novel junction configurations as an extension of our established focused ion beam-based low T c SNS junction fabrication technique [1]. By milling a circular trench (diameter 1 μm, width 50 nm) in a 125 nm Nb 75 nm Cu bilayer we define a superconducting island connected to the bulk of the film by a normal metal barrier and entirely enclosed in-plane by the superconducting film. The properties of this Corbino geometry Josephson junction can be probed by depositing an insulating layer over the device and drilling a 0.3 μm diameter hole down to the island to allow a Nb via to be deposited. Behavior of such devices has been studied in a Helium bath at 4.2 K. An SNS-like current–voltage characteristic and Shapiro steps are observed. It is in terms of magnetic field behavior that the device exhibits novel characteristics: as the device is entirely enclosed in type II superconductor, when a magnetic field is applied perpendicular to the plane of the film, only quantized flux can enter the junction. Hence as applied magnetic field is increased the junction critical current is unchanged, then abruptly suppressed as soon as a flux quantum enters.