Vortex motion and pinning in high temperature superconductors
Vortex motion and pinning in high temperature superconductors
An investigation has been carried out into the magnetic properties of the high-temperature superconductor Bi2Sr2CaCu2O8+ y. Hysteresis measurements have been carried out on powdered samples of Bi2Sr2CaCu2O8+ y, and the critical current density Jc determined from the width of the hysteresis loop. The dependence of the loop width on the field sweep-rate has been investigated. Initially this was explained using a diffusion model of vortices in a molten flux lattice, but further investigation showed that a flux creep model in which the moving bundle volume increases with field and temperature provides a better, though still not complete, description. A complete description can be obtained by combining the two models. The vortex pinning force was measured as a function of field and temperature for three series of rare-earth doped Bi2Sr2CaCu2O8+ y samples, two series of Bi2Sr2(Ca1-xYx)Cu2O8+ y and one of Bi2Sr2(Ca1-xGdx)Cu2O8+ y. The field dependence, which had been thought to indicate a melting of the vortex lattice, was found to be describable using the flux-creep model. Relaxation measurements were carried out to determine the activation energy for the vortex motion in the yttrium-doped series. Doping with rare-earths was found to reduce the pinning force considerably, this is considered to be due to a reduction in superconducting carrier density. Some of the results showed non-systemic behaviour, indicating that other mechanisms affected the strength of the pinning force, and these were investigated. The phase diagram of Bi2Sr2CaCu2O8+ y in the region of the irreversibility line has been investigated. The reduction in Jc was found to be explainable using the flux creep theory. In particular the Lindemann criterion for flux-lattice melting was found to be inapplicable. Measurements of the penetration depth in yttrium and gadolinium doped samples were made using muon-spin rotation spectroscopy. The penetration depth was found in both cases to increase linearly with dopant concentration, consistent with the reduction in carrier density due to doping. The interpretation of the data from the muSR experiments was performed in two distinct ways. Firstly the analysis was performed assuming a Gaussian field distribution inside the sample, from which the penetration depth can be calculated. Secondly the raw data was analysed using the maximum entropy method to determine the field distribution in the sample. This was found to be anisotropic, and to match closely the theoretical prediction made for a powdered 2D superconductor with random orientation.
University of Southampton
1992
Rose, Richard Alexander
(1992)
Vortex motion and pinning in high temperature superconductors.
University of Southampton, Doctoral Thesis.
Record type:
Thesis
(Doctoral)
Abstract
An investigation has been carried out into the magnetic properties of the high-temperature superconductor Bi2Sr2CaCu2O8+ y. Hysteresis measurements have been carried out on powdered samples of Bi2Sr2CaCu2O8+ y, and the critical current density Jc determined from the width of the hysteresis loop. The dependence of the loop width on the field sweep-rate has been investigated. Initially this was explained using a diffusion model of vortices in a molten flux lattice, but further investigation showed that a flux creep model in which the moving bundle volume increases with field and temperature provides a better, though still not complete, description. A complete description can be obtained by combining the two models. The vortex pinning force was measured as a function of field and temperature for three series of rare-earth doped Bi2Sr2CaCu2O8+ y samples, two series of Bi2Sr2(Ca1-xYx)Cu2O8+ y and one of Bi2Sr2(Ca1-xGdx)Cu2O8+ y. The field dependence, which had been thought to indicate a melting of the vortex lattice, was found to be describable using the flux-creep model. Relaxation measurements were carried out to determine the activation energy for the vortex motion in the yttrium-doped series. Doping with rare-earths was found to reduce the pinning force considerably, this is considered to be due to a reduction in superconducting carrier density. Some of the results showed non-systemic behaviour, indicating that other mechanisms affected the strength of the pinning force, and these were investigated. The phase diagram of Bi2Sr2CaCu2O8+ y in the region of the irreversibility line has been investigated. The reduction in Jc was found to be explainable using the flux creep theory. In particular the Lindemann criterion for flux-lattice melting was found to be inapplicable. Measurements of the penetration depth in yttrium and gadolinium doped samples were made using muon-spin rotation spectroscopy. The penetration depth was found in both cases to increase linearly with dopant concentration, consistent with the reduction in carrier density due to doping. The interpretation of the data from the muSR experiments was performed in two distinct ways. Firstly the analysis was performed assuming a Gaussian field distribution inside the sample, from which the penetration depth can be calculated. Secondly the raw data was analysed using the maximum entropy method to determine the field distribution in the sample. This was found to be anisotropic, and to match closely the theoretical prediction made for a powdered 2D superconductor with random orientation.
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Published date: 1992
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Local EPrints ID: 461736
URI: http://eprints.soton.ac.uk/id/eprint/461736
PURE UUID: e5a81d4a-02c3-48c0-9faa-88371c7a97c6
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Date deposited: 04 Jul 2022 18:53
Last modified: 04 Jul 2022 18:53
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Author:
Richard Alexander Rose
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