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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2016)

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.

    Cover page: Focused Helium Beam Irradiated Josephson Junctions
    • Article
    • Peer Reviewed
    UC Riverside Previously Published Works (2019)
      Cover page: Measurement of Magnetic Nanoparticles Using High Transition Temperature Superconducting Quantum Interference Devices
      • Article
      • Peer Reviewed
      UC Riverside Previously Published Works (2018)
      Direct write patterning of high-transition temperature (high-T C) superconducting oxide thin films with a focused helium ion beam is a formidable approach for the scaling of high-T C circuit feature sizes down to the nanoscale. In this letter, we report using this technique to create a sensitive micro superconducting quantum interference device (SQUID) magnetometer with a sensing area of about 100 × 100 μm2. The device is fabricated from a single 35-nm thick YBa2Cu3O7- δ film. A flux concentrating pick-up loop is directly coupled to a 10 nm × 20 μm nano-slit SQUID. The SQUID is defined entirely by helium ion irradiation from a gas field ion source. The irradiation converts the superconductor to an insulator, and no material is milled away or etched. In this manner, a very narrow non-superconducting nano-slit is created entirely within the plane of the film. The narrow slit dimension allows for maximization of the coupling to the field concentrator. Electrical measurements reveal a large 0.35 mV modulation with a magnetic field. We measure a white noise level of 2 μΦ0/Hz1∕2. The field noise of the magnetometer is 4 pT/Hz1∕2 at 4.2 K.
        • Article
        • Peer Reviewed
        UC Riverside Previously Published Works (2019)
        In this work, we investigate two-dimensional arrays of High-T C superconducting quantum interference devices (SQUIDs) for optimization of their electrical transport characteristics. Specifically, we look at devices with different electrode configurations in between the series segments to gain insight into how the array spacing, in the direction of the bias current, affects the voltage magnetic field characteristics. Our results suggest that for spacing dimensions greater than the penetration depth interactions are minimal. Furthermore, comparisons of voltage field characteristics reveal higher modulation voltages and narrower peaks with as the numbers of SQUIDs in the parallel direction increases from 1 to 6. For larger numbers of SQUIDs in parallel greater than 6 little change is observed. These results suggest a pathway to SQUID array scaling for very large numbers of SQUIDs within in a small area.
          Cover page: Investigation of Arrays of Two-Dimensional High-$T_\text{C}$ SQUIDs for Optimization of Electrical Properties
          • Article
          • Peer Reviewed
          UC Riverside Previously Published Works (2019)
          Focused helium ion beam material modification is ideally suited for the fabrication of YBa2Cu3O7-δ (YBCO) Josephson junctions and superconducting feature sizes down to the nanoscale. We report the fabrication and measurement of YBCO nano-slit SQUIDs and study how scaling the dimensions of the SQUID body and electrodes influences the electrical properties. Sixteen nano-slit SQUIDs with different width and length were fabricated from a single-layer 25-nm-thick YBCO film. The experimental results yield an estimation of the sheet inductance to be ∼4 pH/□ and a penetration depth of ∼180 nm. The temperature dependence of the inductance agree well with an impurity scattering model, indicating that the TC of the film was ∼82 K, and the ratio between geometric inductance and kinetic inductance at 0 K was ∼0.8. These results provide solid basis for the design of high-performance, high-TC micro-SQUID magnetometers.
            Cover page: Inductance Investigation of YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7−δ</sub> Nano-Slit SQUIDs Fabricated With a Focused Helium Ion Beam
            • Article
            • Peer Reviewed
            UC Riverside Previously Published Works (2019)
            High-TC superconductor thin film heterostructures were deposited using reactive co-evaporation for dual layer electronic applications. The epitaxial structure consisted of 35-nm YBa2Cu3O7-δ(YBCO), 75-nm CeO2 , 150-nm YBCO, and 20-nm CeO2 on r-plane sapphire wafers. The critical temperature was measured to be 83.6 K and 84.8 K for the bottom and top YBCO layers, respectively. Atomic force microscopy reveals smooth surfaces with RMS roughness of the top YBCO layer to be 4.7 nm. The CeO2 insulating layer exhibited hopping conduction that freezes out at low temperature, making these structures suitable for electrical circuits with isolated ground planes.
              Cover page: YBa$_2$Cu$_3$O$_{7-\delta }$-CeO$_2$-YBa$_2$Cu$_3$O$_{7-\delta }$ Multilayers Grown by Reactive Co-Evaporation on Sapphire Wafers
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