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A magneto-optic investigation of flux penetration in high temperature superconductors Gardner, Michael W.

Abstract

An efficient non-destructive magneto-optic imaging system was developed that employs special iron-garnet indicator films to image magnetic fields in high temperature superconductors. The flexibility of the system allows the investigation of many different types of samples having different sizes and characteristics. Two very high quality, low pinning, thin, flat YBa₂Cu₃O₆.₉₅ single crystals are imaged using this system to observe field penetration dynamics. Slightly different pinning characteristics of both samples enable a comparison to be made between the different flux penetration dynamics observed. In one ultra low pinning crystal, penetrating flux is found to accumulate in the sample's central regions while avoiding the inner edges and in the other higher pinning crystal flux tends to initially avoid both the central regions and sample's inner edges. These results compare favourably with a theoretical model of Zeldov et al.[21] that characterizes penetration phenomenon in low pinning superconductors. Field penetration of a Tl₂Ba₂CaCu₂ thin film is also observed for different sample temperatures up to the sample's superconducting transition temperature. A new computational technique is employed to calculate the critical current of the film using imaged field strengths. A comparison of the imaged fields and calculated currents is made with a theoretical model devised to characterize field penetration in superconductors having a thin circular geometry. From these results the critical current temperature dependence is established and is fitted to a simple phenomenon logical model. A small discontinuity in the data 10 K below the sample's transition temperature is also noted and is examined.

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