Impedance control of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub x/ Josephson junctions on MgO substrates for millimeter wave sensing

SIS-type (Superconductor-Insulator-Superconductor) intrinsic stacked Josephson junctions (ISJJs) and SNS-type (Superconductor-Normal conductor-Superconductor) step-edge Josephson junctions (SEJJs) have been successfully fabricated using anisotropic Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub x/ thin films on MgO substrates. In order to apply them to millimeter wave receivers, their normal resistance (Rn) was controlled, which determines the impedance matching for millimeter wave signals. For electrical transport along the c-axis, the Rn of a single unit ISJJ with a 5 /spl mu/m-square mesa was estimated to be 580 /spl Omega/. For electrical transport parallel to the ab-plane, the R/sub n/ of series connected six SEJJs (6-SEJJs) with the gentle slope of 28/spl deg/ was 2.0 to 3.8 /spl Omega/. Since their responsivities as millimeter wave detectors were two to three orders of magnitude larger than those of single SEJJs, efficient IF (intermediate frequency) output of fundamental mixing with LO (local oscillation) power of a few microwatts was observed up to 95 K.