Step edge Josephson junctions and high temperature superconducting quantum interference device (SQUID) gradiometers

This thesis is concerned with the development of Superconducting Quantum Interference Device (SQUID) gradiometers based on the high temperature superconductor YBa_2Cu_3O_7_-_#delta# (YBCO). A step-edge Josephson junction fabrication process was developed to produce sufficiently steep (>60 deg) step-edges such that junctions exhibited RSJ-like current-voltage characteristics. The mean I_CR_N product of a sample of twenty step-edge junctions was 130#mu#V. Step-edge dc SQUIDs with inductances between 67pH and 114pH were fabricated. Generally the SQUIDs had an intrinsic white flux noise in the 10-30#mu##PHI#_0/#sq root#Hz range, with the best device, a 70pH SQUID, exhibiting a white flux noise of 5#mu##PHI#_0/#sq root#Hz. Different first-order SQUID gradiometer designs were fabricated from single layers of YBCO. Two single-layer gradiometer (SLG) designs were fabricated on 10x10mm"2 substrates. The best balance and lowest gradient sensitivity measured for these devices were 1/300 and 308fT/cm#sq root#Hz (at 1 kHz) respectively. The larger baseline and larger flux capture area of the pick-up loops in a large area SLG design, fabricated on 30x10mm"2 substrates, resulted in significant improvements in the balance and gradient field sensitivity with 1/1000 and 50fT/cm#sq root#Hz (at 1kHz) measured respectively. To reduce the uniform field effective area of SLOs 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 2#mu#m"2 - 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 were fabricated on 10x10mm"2 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 was approximately 1/5000 - an order of magnitude better than the balance of similar SLGs incorporating narrow-linewidth SQUIDs. (author)SIGLEAvailable from British Library Document Supply Centre- DSC:DXN056128 / BLDSC - British Library Document Supply CentreGBUnited Kingdo