Shunting YBa₂Cu₃O_7-x Josephson Junctions and Arrays Fabricated by He-FIB

Fabrication of YBa₂Cu₃O_7-x Josephson junctions (JJs) using a direct-write helium-focused helium ion beam (He-FIB) technique has great potential for various applications due to tunable JJ parameters and arbitrary positioning. This technique allows placing multiple JJs in series to create JJ arrays. They could be used as a voltage standard that is operated at higher temperatures and driving frequencies than the currently used niobium-based JJ arrays. Small YBa₂Cu₃O_7-x He-FIB JJ arrays with superposed Shapiro steps for up to three JJs have already been demonstrated. The number of JJs in the array was limited by the parameter spread of the individual JJs. This study presents WRspice simulations of JJ arrays, which indicate that the normal state resistance R_n is the most crucial parameter to achieve flat Shapiro steps in JJ arrays. The idea of adding a shunt resistor to the JJ array to deal with the spread in R_n values is investigated. A new design for GHz frequencies is presented, and the fabrication process is described. The properties of the shunt resistor are evaluated by van der Pauw measurements. Comparison of measurements with and without shunt resistor shows the desired effect of changing the total resistance of the JJ array while maintaining the critical current value. Finally, measurements of JJ arrays demonstrating superposed Shapiro steps at 20 k at GHz frequencies are presented.