YBa₂Cu₃O₇ nm-sized constriction-type Josephson junctions and nanoSQUIDs fabricated by focused helium ion beam irradiation

We report on the fabrication and investigation of constriction-type Josephson junctions (cJJs) in YBa₂Cu₃O₇ (YBCO) thin films by irradiation with a focused helium ion beam (He-FIB). He-FIB irradiation locally modifies the properties of the YBCO films and, depending on irradiation dose, allows us to write insulating or normal conducting areas and lines, including Josephson barriers. We investigated the crystal structure by transmission electron microscopy and electric transport properties of He-FIB-patterned areas. Applying an optimized dose preserves the YBCO crystal structure but suppresses superconductivity. Thus, such areas can be used as shunt resistors because of their ohmic current-voltage characteristics (IVCs). With this approach we created ultra-narrow cJJs between resistive triangular areas. Our smallest cJJs have constriction widths of only a few nm and show resistively-shunted-junction (RSJ) like IVCs with characteristic voltages of several mV at 4.2 K. Embedding two cJJs in a superconducting ring with an amorphized hole also created by He-FIB, we prepared nanoSQUIDs with a transfer function of several mV per flux quantum at 4.2 K. Thus, our approach demonstrates the feasibility of using He-FIB irradiation for editing YBCO Josephson circuits on the nm scale.