LARGE SHAPIRO STEPS AND WIDE JUNCTION BEHAVIOR IN HTS JOSEPHSON-JUNCTIONS AT NITROGEN TEMPERATURES

We have fabricated bicrystal high-T-c superconductor (HTS) Josephson j unctions which, under microwave radiation, exhibited first current ste ps (1. Shapiro steps) with an amplitude of 1 mA at 80 K. This was poss ible because of a considerable increase of the critical current I-c of the junctions by increasing the HTS film thickness up to 700 nm. Howe ver, above a certain I-c threshold, the junction behaviour deviated fr om the simple resistively shunted junction (RSJ) model. We show that t he junctions can be described by the wide junction model. Both the sta tic (dependence of I-c on magnetic field) and the dynamic characterist ics (response to microwave radiation) of our junctions can be well fit ted in the frame of the wide junction model. We present data of juncti ons with relative widths w/lambda(j) = 5 and 7 (w-physical width of th e junction, lambda(j)-Josephson penetration depth), i.e. with w/lambda (j) lying in the intermediate interval where the behavior of the junct ions is most sensitive to changes in w/lambda(j). This allowed us to f it experimental data with relatively good accuracy. We discuss the con sequences of wide junction behavior on the maximal first Shapiro step and the impact on metrological applications of HTS Josephson junctions . (C) 1998 Elsevier Science Ltd. All rights reserved.