Influence of La-doping of YBa₂Cu₃O₇ on transport properties of interface-engineered ramp-edge junctions

We have investigated the influence of La-doping of the YBa_2-x La_xCu₃O₇ (YBCO) thin film electrodes on dc and ac Josephson properties of ramp-edge junctions with interface-engineered barriers. Non-doped optimized junctions exhibit critical current densities, j_c, of 5·10⁴ A/cm² and normal sheet resistances, R_N·A, of 3·10^-9 Ohm·cm² at 77 K. The transport properties of junctions with x=0.03 La-doping are quite similar to those of non-doped junctions. La-doping of x=0.05 and x=0.07 leads to an increase of R_N·A and decrease of j_c by about one order of magnitude. Devices with j_c<10 ⁴ A/cm², which are operating in the short junction limit exhibit current-voltage characteristics without any excess current and 100% modulation of critical current in external magnetic field. The current voltage characteristics exhibit well defined Shapiro steps. The dependence of current-step height on microwave current can be described in the resistively-shunted junction model. The strong changes in the temperature dependencies of j_c and R_N·A suggest different electrical transport properties for junctions fabricated with non-doped and La-doped YBCO. The temperature dependence of the critical current and the normal resistance allows us to draw conclusions to the transport properties of our junctions