Influence of La-doping of YBa/sub 2/Cu/sub 3/O/sub 7/ on transport properties of interface-engineered ramp-edge junctions

We have investigated the influence of La-doping of the YBa/sub 2-x/La/sub x/Cu/sub 3/O/sub 7/ (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/sub c/, of 5/spl middot/10/sup 4/ A/cm/sup 2/ and normal sheet resistances, R/sub N//spl middot/A, of 3/spl middot/10/sup -9/ Ohm/spl middot/cm/sup 2/ 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/sub N//spl middot/A and decrease of j/sub c/ by about one order of magnitude. Devices with j/sub c/<10/sup 4/ A/cm/sup 2/, 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/sub c/ and R/sub N//spl middot/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.