Millimeter microwave properties of high-temperature superconductors
The broad objective of this experimental study is to characterize the microwave properties of high temperature superconductors in the presence of strong magnetic fields. The dissertation primarily describes the development of a microwave system (35 GHz) that measures trasmitted and reflected power fractions of thin superconducting films, and the analysis techniques used to interpret the results. YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) films deposited on LaAlO{sub 3} substrates are investigated, with the microwave propagation direction and magnetic field both perpendicular to the film and parallel to the crystalline axis. Results show no hysteresis on dependence on incident power level, and are changed markedly when power impinges on the substrate instead of the film. Two analysis approaches are implemented, both of which require the solution of the problem of wave propagation through a bilayer structure. The first method employs Coffey-Clem theory for the electrodynamic response of the superconductor, focusing on losses due to vortex motion. A gradient line search of the multidimensional parameter space yields optimal theory parameters. The second method employs direct numerical inversion of the abovementioned wave solution via the two dimensional Newton-Raphson algorithm. Complete results are given for a YBCO film in terms of the complex penetration depth as a function of temperature and field (80-95K, 0-5.4 tesla), revealing substantial increase due to the applied field. Supplemental far-infrared transmission measurements for YBCO exhibit only minimal magnetic field effects, implying that the dominant dissipation processes are probably not the same as at microwave frequencies. Results from a microwave resonant cavity perturbation method sensitive to changes in surface resistance are also discussed, for the conventional type-II superconductor Pb{sub 90}Bi{sub 10} and high temperature superconductors YBCO and Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub x}.
- Research Organization:
- Notre Dame Univ., IN (United States)
- OSTI ID:
- 106818
- Resource Relation:
- Other Information: TH: Thesis (Ph.D.); PBD: 1994
- Country of Publication:
- United States
- Language:
- English
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