Fabrication and characterization of high-T_c YBa ₂Cu₃O_7-x nanoSQUIDs made by focused ion beam milling

We have fabricated high-T_c nanoscale superconducting quantum interference devices (nanoSQUIDs) with a hole size of 250 nm × 250 nm based on a 100 nm bridge at 77 K by focused ion beam milling and ion implantation. At 78 K, the curve of the voltage branch became roughly linear and agreed with the Josephson-like behavior. The sample exhibited strong flux flow behavior at temperatures under 76 K. The voltage flux characteristic curves, V -I_mod, of the nanoSQUID at different bias currents at 78 K were observed. Typically, critical currents of 15 νA and peak-to-peak values of the voltage flux transfer function of 3.7 νV were measured. The measured data strongly suggest that the weak link structure could be a superconducting metal with a critical temperature T_c′ smaller than that (T _c) of other YBa₂Cu₃O_7-x (YBCO) films. This fabrication method of combining a nanobridge and ion implantation can improve the yield of nanojunctions and nanoSQUIDs.