Fabrication of Inductively-Coupled Double-Josephson Junctions Using High-Temperature Superconductors

We fabricated double interface-treated Josephson junctions with vertically-stacked geometry based on the high temperature superconductor YBCO. The junctions have a superconductor-insulator-superconductor-insulator-superconductor structure. If the intermediate layer is thin enough, the two junctions are coupled inductively through the intermediate layer, and the two junctions switch simultaneously. The double junctions with 30 nm thick intermediate layer exhibited resistively-shunted-junction characteristics and had no kinks in their I - V curves. In a typical chip, an average of the critical current density was 1.65 kA/cm² with 1 sigma spread of 14.6%. The nominal I_cR_n product for a double junction was 2.43 mV with 1 sigma spread of 5.72%. This I_cR_n product is 1.5-2 times larger than that of the single vertically-stacked junction having similar critical current density. These experimental results suggest the simultaneous switch of the two junctions. We expect that higher nominal I_cR_n products would be obtained after choosing appropriate process parameters, because the I_cR_n products of our previous single junctions have reached 2 mV.