High-temperature superconductor Josephson junctions with a gradient Pr-doped Y_1-xPr_xBa₂Cu₃O _7-δ (x=0.1, 0.3, 0.5) as barriers

High-temperature superconductor YBa₂Cu₃O _7-δ based superconducting-normal-superconducting (SNS) Josephson junctions were fabricated using a unique device design. The normal material included a gradient Pr-doped Y_1-xPr_xBa ₂Cu₃O_7-δ layer which was composed of a light doping (x=0.1) next to both YBa₂Cu₃O _7-δ electrodes, a slightly higher doping (x=0.3) towards the center, and a doping concentration of x=0.5 in the middle of the N layer. This design graded the lattice mismatch between YBa₂Cu₃O _7-δ and the N layer, thus avoiding the accumulation of all the lattice strain at one interface. It also results in good chemical, thermal, and structural compatibility between adjacent layers for the desired multilayer structures. The SNS junctions fabricated in this way showed resistively shunted junction current-voltage characteristics under dc bias and Shapiro steps under microwave irradiation at a temperature in the range of 75-87 K. Direct current superconducting quantum interference devices showed a voltage modulation about 5 μV by a magnetic field at liquid nitrogen temperature.