Study of HTS Nanobridge Josephson Junctions Made by FIB

Artificial bicrystal junctions are often used for Josephson junctions in high-temperature superconductor superconducting quantum interference devices (SQUIDs). However, the junctions using bicrystal substrates are expensive and have large variations in their characteristics. Furthermore, the placement of SQUIDs on substrate is limited to only linear bicrystal junctions. On the other hand, the superconductor-constriction-superconductor (ScS) junction is a technique that is used to control the critical current of a superconductor thin film by focused ion beam (FIB) irradiation. We have fabricated ion damage junctions and nanobridge junctions, which are a kind of ScS junction, by FIB processing of YBa ₂ Cu ₃ O _{7- δ} (YBCO) thin films. The ion damage junctions are fabricated by line scanning of YBCO thin films at normalization fluence, which sufficiently reduces the critical current. The nanobridge junctions are fabricated by narrowing the width of YBCO thin films using FIB. SQUIDs were fabricated to demonstrate the operation of both types of Josephson junctions. The nanobridge type showed relatively a larger modulation amplitude.