Josephson Junctions and Devices fabricated by Focused Electron Beam Irradiation

摘要

The irradiation of high Tc superconducting thin films with a focused electron beam, suchas that obtained in a scanning transmission electron microscope (STEM), can result in theformation of a Josephson junction. The conditions required for the formation of theseJosephson junctions in YBa2Cu3O7-d and related compounds are discussed as well as thephysical properties of the irradiated material. From electrical transport measurements ofindividual Josephson junctions it was found that these junctions have aSuperconductor/Normal/Superconductor (SNS) nature. Low temperature anneal studiesindicate that Josephson junctions with optimum properties can be obtained by acombination of a high electron dose and subsequent low temperature anneal. Extremelyhigh electron doses resulted in the formation of a purely resistive region. The electricaltransport in such regions with a dimension of 200 nm in the direction of current transportis shown to be compatible with variable range hopping (VRH). Barriers with the samelength but a finite superconducting transition temperature showed a low bias resistancethat is significantly lowered due to proximity coupling. Using purely resistive regions incombination with Josephson junctions, devices consisting of two closely spacedJosephson junctions with a third terminal connected to the shared electrode werefabricated and characterised (minimum separation 20 nm). The distinct behaviour of theJosephson critical current with applied magnetic field (Ic(B)) of these devices was foundto be well described by a newly developed model, which incorporates the effect of thestatic redistribution current in the shared electrode on the phase distribution of theJosephson junctions. An important finding is that the behaviour of the high critical currentwith applied magnetic field of two closely spaced junctions was found to be consistentwith a model system consisting of a closely spaced Josephson junction and a resistivebarrier. A three terminal device with Josephson junctions at small separations was foundto have a significantly increased transresistance when compared with the individualresistance of the Josephson junctions it constituted of. A number of illustrative examplesof device structures realised with the focused electron beam irradiation technique are alsoincluded.