A magneto-optical apparatus has been designed to investigate flux and current distributions in various forms of high-temperature superconductor after the application of external magnetic fields, transport currents, and both in sequence. It is concluded that flux motion depends significantly on the superconductor characteristic flux pinning strength and geometry. The flux profiles at the surface of an isolated thin film strip, and thin films in x-array and shunted-x-array arrangements have been studied in detail. It has been found that the theoretical flux profile, characterised by a logarithmic singularity at its edges and a vertical slope at the flux front, become rounded when the sample thickness is taken into account. Experiment has confirmed that the current density reaches its critical value in the flux-penetrated region while there is a continuous non-zero current density in the central flux-free region. Also magnetic interactions exist between the strips. Band-shaped and dome-shaped flux distributions caused by geometrical barrier at different weak pinning strengths were observed in Bi2Sr2CaCu2O8+x single crystals between 20 K and 70 K. The geometrical barrier also governs the transport current leading to a square-shaped distribution near the crystal edges at 20 K. Cracks and defects in Bi-2223/Ag tapes were visualised without destruction of the tape. The inhomogeneous flux distribution indicated that defects, including cracks, in superconducting filaments limit the current-carrying capacity of the tape. In addition, at 70 K a fraction of the transport current has been found to flow in the silver sheath. An investigation of the influence of the twin planes on vortex motion in a YBa2Cu3O7-d single crystal has shown that after a twin barrier is initially overcome, the twin planes, regardless of their directions, act as an easy-flow channel for vortex penetration at low temperatures as the pinning strength in the twin planes is weaker than that in the untwinned region, whereas, they act as a barrier at high temperatures where the pinning strength in the twin planes is stronger. Furthermore, vortex penetration along the twin planes is a factor of four greater than that for the untwinned region at 25 K.
| Identifer | oai:union.ndltd.org:ADTP/225441 |
| Date | January 2000 |
| Creators | Lin, Zhi Wei, Physics, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Physics |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Zhi Wei Lin, http://unsworks.unsw.edu.au/copyright |
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