Properties of superconducting weak links formed by magnetically weakening a short length of a uniform aluminum film

The Josephson effect has been studied in uniform aluminum films in which a section about a coherence length long has been weakened by the application of a perpendicular magnetic field. These films, which are about 100 μm long, have a constant thickness and width (about 0.04 μm × 10 μm). The applied field is confined to a strip across the film about 0.5 μm in length by lead shield films which are deposited on top of the Al film but are electrically insulated from it. The pronounced Josephson-like features which we observe generally appear for applied fields in the shield gap approximately equal to or greater than Hc2for a similar uhshielded Al film. They persist to applied fields several times Hc2. While we do not observe the Josephson effect for fields much below Hc2this may be a result of heating associated with the higher critical currents at low fields rather than an intrinsic property of the film. We note that heating generally prevents the observation of any Josephson effect in planar Al film microbridges. The magnetically weakened junctions have the unusual property that their resistance R, which is measured at small current in the high field limit, is a strong function of temperature. R diverges near Tcapproaching the normal state resistance of the entire strip a few millidegrees below the onset of the zero field resistive transition.