We present strong evidence that helical flux lines can concurrently, enter and leave the wall of hollow cylinders of sintered high superconductors at T$\sb{\rm c}$ 77 K, hence cut and traverse each other. Tubes of two different materials, denoted BiSCCO and YBCO, have been studied. The evidence for flux line cutting and cross-flow is examined in the perspective of basic concepts. The traffic of the flux lines is also described formally. The penetration fields across the wall and into the grains of the sintered ceramic tubes are determined from their magnetic response to an applied axial field H$\sb{//}$ and from their flux trapping behaviour when subjected to two standard procedures, denoted H$\sb{\rm cool}$ and H$\sb{\rm cycle}$. This provides information on j$\sb{\rm c\perp m}$ and j$\sb{\rm c\perp g}$, the intergrain and intragrain critical current densities for depinning of flux lines. The penetration fields are seen to correlate with salient features (peaks and valleys) of the flux line cutting and cross flow phenomena. This enables us to claim that we are witnessing flux line cutting and cross flow in the weak link regime in the BiSCCO tube and in this regime as well as in the interior of the grains in the YBCO tube. The Clem/Perez-Gonzalez phenomenological theory is exploited in a simplified framework to describe the crucial features of the data semi-quantitatively. This analysis confirms the above conclusions and provides estimates of j$\sb{\rm c//m}$ and j$\sb{\rm c//g}$, the critical current densities for intergranular and intragranular flux line cutting and their dependence on the magnetic flux density.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/6904 |
| Date | January 1993 |
| Creators | Celebi, Selahattin. |
| Contributors | LeBlanc, Marcel A. R., |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 171 p. |
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