Authors: B. Müller, M. Karrer, F. Limberger, M. Becker, B. Schröppel, C.J. Burkhardt, R. Kleiner, E. Goldobin, and D. Koelle
Physical Review Applied volume 11, 044082 (2019)
Abstract: By scanning with a 30–keV focused He ion beam (He-FIB) across YBa2Cu3O7 (YBCO) thin-film microbridges, we create Josephson barriers with critical current density jc adjustable by irradiation dose D. The dependence jc(D) yields an exponential decay. At 4.2K, a transition from flux-flow to Josephson behavior occurs when jc decreases below approximately 2MA/cm2. The Josephson junctions exhibit current-voltage characteristics (IVCs) that are well described by the resistively and capacitively shunted junction model, without excess current for characteristic voltages Vc≲1mV. Devices on MgO and LSAT substrates show nonhysteretic IVCs, while devices on SrTiO3 show a small hysteresis. For all junctions, an approximate scaling Vc∝j1/2c is found. He-FIB irradiation with a high dose produces barriers with jc=0 and high resistances of 10kΩ to 1GΩ. This provides the possibility to write highly resistive walls or areas into YBCO using a He-FIB. Transmission electron microscopy reveals an amorphous phase within the walls, whereas for lower doses the YBCO stays crystalline. We have also “drawn” superconducting quantum-interference devices (SQUIDs) by using a He-FIB for the definition of the SQUID hole and the junctions. The SQUIDs show high performance, with flux noise <500nΦ0/Hz1/2 in the thermal white-noise limit for a device with 19pH inductance.