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Focused Helium Beam Irradiated Josephson Junctions

  • Author(s): Cho, Ethan
  • Advisor(s): Dynes, Robert C
  • et al.

In this thesis, I studied the superconductor-insulator transition in thin film planar YBa2Cu3O7-δ (YBCO) Josephson junctions with focused helium ion beams(FHB). Josephson junctions, patterned by a direct-write focused helium beam with a beam size of 500 pm, have a barrier width on the order of the quasiparticle tunneling length. By increasing the barrier strength with an irradiation dose that creates disorder in the material, the junction transitions continuously from a superconductor-normal metal-superconductor (SNS) to superconductor-insulator-superconductor (SIS). As described in model by Blonder, Tinkham, and Klapwijk (BTK), the transport mechanism shifts from Andreev reflection to tunneling. The product of critical current (IC) and normal state resistance (RN) is larger as a result of the higher resistance of the junction. Using high resistance SIS junctions, we measured the dynamic conductance of YBCO-YBCO junctions in the conduction plane. The peak of the dynamic conductance agrees with the reported values of the energy gap in the literature. In addition, the temperature dependence of the dynamic conductance peak fitted well with the temperature dependence of the energy gap in the Bardeen-Cooper-Scherifer (BCS) theory.

In the view of fabrication technology, an FHB can be used as a lithography tool to pattern circuits. By applying higher irradiation dose, insulating barriers were created, defining the current paths and junction widths. Nano-scale features, which were difficult if not impossible with previous processing techniques, were directly written with an FHB. The smallest feature made with direct patterning with an FHB is 20 nm wide junction. Also I observed hysteretic current-voltage characteristics, from junctions with the Stewart-McCumber parameter βC greater than 1. Nanopatterning with an FHB allows control over junction parameters of IC, RN, ICRN, and βC that extended the parameter space of the junctions. With nanojunctions, superconducting electronics could be made with very high normal state resistance reaching several hundred ohms and be impedance matched with semiconductor electronics.

High-transition temperature superconductors have order parameters with some d-wave symmetry. I observed angular variation in IC, RN, and ICRN of junctions in different orientations that have a similar pattern to the d-wave pattern. This experiment was conducted with YBCO-YBCO junctions, as opposed to different material interfaces that do not lie in the conduction plane. Junctions in certain directions exhibit ICRN over 1 mV.

I also made superconducting quantum interference devices (SQUIDs) of FHB SIS and SNS junctions. A single SIS SQUID exhibits voltage modulation of 200 μV at 4 K. The SNS SQUID at 68 K has a flux noise of 10 μΦ0/Hz-1/2 in white noise range and 20 μΦ0/Hz-1/2 at 1 Hz, on par with the current state-of-the-art HTS SQUID in the white noise range and out performs it below 10 Hz. For most biomedical imagining applications, the signals below 10 Hz suggest HTS SQUID sensors could be an improvement to current technologies. Lastly, 1D arrays of long Josephson junctions could potentially be an alternative for applications that require large dynamic range. For a 10 micron-wide, six hundred series junctions array, the voltage modulation was 23 mV, appearing linear over 12 mV and 30 μT, with a slope or transfer function of 500 V/T. As the junctions widens, the V--B skews more. In principle, with proper design of the junction width, the transfer function can be much larger.

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