Electrical properties of YBCO/Ca-YBCO/YBCO trilayer Josephson junctions using c-axis oriented films

We have studied electrical properties of YBCO/Y/sub 0.7/Ca/sub 0.3/Ba/sub 2/Cu/sub 3/O/sub 7-x/ (Ca-YBCO)/YBCO trilayer junctions using c-axis oriented films. The Ca-YBCO films used here are the overdoped version of YBCO and show transition temperatures around 55 K. All of the junctions show RSJ-like current-voltage characteristics in a certain temperature range, though the range decreases with the Ca-YBCO interlayer thickness. For an applied magnetic field, the junctions show Fraunhofer pattern, indicating a Josephson coupling through the interlayers. The normal resistance of the junctions is proportional to the thickness of the interlayer, suggesting little interface resistance. The exponential dependence of the critical current on temperature and that on the interlayer thickness mean that the junctions behave as SNS junctions. These experimental results imply the proximity effect along c-axis in high temperature superconductors.