Large Shapiro steps and wide junction behaviour in HTS Josephson junctions at nitrogen temperatures

Abstract

Abstract We have fabricated bicrystal high-Tc superconductor (HTS) Josephson junctions which, under microwave radiation, exhibited first current steps (1. Shapiro steps) with an amplitude of 1 mA at 80 K. This was possible because of a considerable increase of the critical current Ic of the junctions by increasing the HTS film thickness up to 700 nm. However, above a certain Ic threshold, the junction behaviour deviated from the simple resistively shunted junction (RSJ) model. We show that the junctions can be described by the wide junction model. Both the static (dependence of Ic on magnetic field) and the dynamic characteristics (response to microwave radiation) of our junctions can be well fitted in the frame of the wide junction model. We present data of junctions with relative widths w/λj=5 and 7 (w-physical width of the junction, λj-Josephson penetration depth), i.e. with w/λj lying in the intermediate interval where the behavior of the junctions is most sensitive to changes in w/λj. This allowed us to fit experimental data with relatively good accuracy. We discuss the consequences of wide junction behavior on the maximal first Shapiro step and the impact on metrological applications of HTS Josephson junctions.