Studies on the Bi System Josephson Junctions with High Criteical Temperature

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Among Bi_2Sr_2Ca_<n-1>Cu_nO_x superconductors, n=3 phase (2223 phase) is an oxide superconductor which has a critical temperature more than 100K.This material has the possibility to realize devices more stable and of better performance than those of Y system superconductor. In this research, on the basis of the thin film technology of 2223 phase, we tried to fabricate the following Josephson junctions.1. Edge-type Junctions :We fabricated edge type junctions using the 2223 phase as the superconducting electrodes and the 2201 phase as the middle layr. Their properties were RSJ-like. It was suggested that the grain boundary at the bottom of the step of the upper electrode also worked as a junction.2. Grain-boundary Junctions :(1) Bicrystal Junctions :We tried to fabricate grain-boundary Josephson junctions using 2223 phase films deposited on bicrystal substrates. Their properties are similar to those of Y system superconductor. On the other hand, the high junction resistances and the low … More critical current were confirmed as characteristic features of the grain-boundary junctions of the 2223 phase. By analyzing the junction characteristics, it was found that there are normal phase layrs near the insulating layr in the junction. The conduction mechanism of the junction was discussed. SQUID's were also fabricated using this bicrystal junction. The critical current was modulated periodicaly by the external magnetic field, and in spite of the small modulation, relative large output valtage was observed reflecting its high dynamic resistance, though we have not evaluated their noise properties.(2) Step-Edge Junctions :We fabricated step-edge type grain-boundary junctions by depositing the 2223 thin film on the MgO step substrate. The current-voltage characteristics was RSJ-like and it was suggested that the grain boundaries were formed at the top and the bottom of the step. Junctions using the films annealed for a short time revealed the improvement of the critical temperature, the critical current and the I_cR_n product of the junction. The possibility that there are normal phase layrs near the insulating layr in the junction was also suggested.3. Intrinsic Junctions :We tried to fabricate the intrinsic junctions using the 2223 phase thin films on the SrTiO_3 step substrates. The gap structure corresponding to the number of the layrs, i. e., the series Josephson junctions was observed. Less