Transition from superconducting-normal metal to superconducting-insulating barrier in focused helium beam YBCO Josephson junctions

We report measurements of the metal--insulator transition (MIT) in high--transition--temperature--superconductor, planar Y--Ba--Cu--O Josephson junctions. The junctions were made by irradiation of a $\sim$1~nm barrier with a focused helium ion beam. To describe the MIT we use a theoretical model by Blonder, Tinkham and Klapwijk (BTK). This model explains transition of the transport at a superconducting normal metal/insulator interface using a single parameter, the barrier strength (Z). In our experiment, the dose of the He$^+$ ions serves the role of Z. We tested 20 junctions created using 20 different doses ranging from $10^{16}$~He$^+$/cm$^2$ to $10^{17}$~He$^+$/cm$^2$. We measured current--voltage characteristics of the junctions for several temperatures, and extract critical current ($I_C$), normal state resistance ($R_N$) and $I_CR_N$ from fits to the data. We show that the $R_N$ of the junctions show metallic behavior for lower doses and change continuously to insulating at higher doses. We have demonstrated precise control over the junction parameters with He$^+$ ion dose, which opens up an avenue for studying materials sensitive to disorder as well as reproducible production of Josephson junctions for application.