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 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 firstorder 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 /5000 - an order of magnitude better than the balance of similar SLGs incorporating conventional narrow-linewidth SQUIDs.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 firstorder 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 /5000 - an order of magnitude better than the balance of similar SLGs incorporating conventional narrow-linewidth SQUIDs