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Title: Step edge Josephson junctions and high temperature superconducting quantum interference device (SQUID) gradiometers
Author: Millar, Alasdair J.
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2002
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This thesis is concerned with the development of Superconducting Quantum Interference Device (SQUID) gradiometers based on the high temperature superconductor YBa2Cu3O7-o (YBCO). A step-edge Josephson junction fabrication process was developed to produce sufficiently steep (> 60°) step-edges such that junctions exhibited RSJ-like current-voltage characteristics. The mean I(RN product of a sample of twenty step-edge junctions was 130jtV. Step-edge dc SQUIDs with inductances between 67pH and 114pH were fabricated. Generally the SQUIDs had an intrinsic white flux noise in the 10-30μ(Do/ Hz range, with the best device, a 70pH SQUID, exhibiting a white flux noise of 5μ4bo/ Hz. Different first-order SQUID gradiometer designs were fabricated from single layers of YBCO. Two single-layer gradiometer (SLG) designs were fabricated on 10 x 10mm2 substrates. The best balance and lowest gradient sensitivity measured for these devices were 1/300 and 308fT/cm Hz (at 1kHz) respectively. The larger baseline and larger flux capture area of the pick-up loops in a large area SLG design, fabricated on 30 x 10mm2 substrates, resulted in significant improvements in the balance and gradient sensitivity with 1/1000 and 50fT/cm Hz (at 1kHz) measured respectively. To reduce the uniform field effective area of SLGs and therefore reduce the direct pick-up of environmental field noise when operated unshielded, a novel gradiometric SQUID (G-SQUID) device was developed. Fabricated from a single layer of YBCO, the G-SQUIDs, with inductances of 67pH, had small uniform field effective areas of approximately 2μm2 - more than two orders of magnitude smaller than the uniform field effective areas of conventional narrow-linewidth SQUIDs of similar inductance. Two designs of G-SQUID SLGs were fabricated on 10 x 10mm2 substrates. Due to their small effective areas, when cooled unshielded these devices showed no increase in their white flux noise. The best balance achieved for a G-SQUID SLG was approximately 1/5000 - an order of magnitude better than the balance of similar SLGs incorporating conventional narrow-linewidth SQUIDs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Solid-state physics Solid state physics Measurement