Estudio Energético de Redes de Vórtices Nanoestructuradas en YBa2Cu3O7 mediante Decoración Magnética

R. F. LUCCAS

Universitat Autònoma de Barcelona

Año: 2015 p. 205

9788469514993

Efforts in superconductivity for new discoveries are pushed up for a vital necessity of application.[1-7] It is know that construction of long superconductor cables have been able due to application of results obtained in lab experiments using YBa2Cu3O7 model systems,[8-12] superconductor material with the best performance. Vortex pinning has been the hot topic for improve superconductor properties, since this effect is the one that produce loose of energy. Vortex lattice structured due to material defects presence point out a vortex-defect interaction necessary for a good vortex pinning and, in consequence, an improvement of superconductor properties. However, evaluation techniques typically used work with the collective behavior of the vortex lattice,[13, 14] with results that barely allow to separate individual contributions from different interactions. This thesis is focused on the vortex lattice structured analysis due to defect presence, allowing a quantitative comparison between different kinds of defects. The analysis is based on the YBa2Cu3O7 flux line lattice behavior in direct interaction with surface artificial defects that can improve superconductor properties of the material. The study uses the Magnetic Decoration technique for the visualization of the flux line lattice in model systems (single crystals and films of YBa2Cu3O7 growth respectively by flux method and liquid phase ephitaxy on MgO substrates). Artificial defects under study are generated using two different techniques, focused ion beam and indentation at nanometric scale. For this study, first of all a montage and set up of the magnetic decoration technique is doing. Then a model for flux line lattice energies estimation is developed based on magnetic decoration results. This model allows evaluating separately individual pinning contributions, comparing for the first time results in a quantitative way. Finally a virtual systems modeling is performed, estimating defect properties due to a direct comparison between real and artificial vortex systems. We point out that this work shows highlights in three different ways. In one hand we have started with a technique for magnetic domains visualization (here used exclusively for vortices in superconductor materials). This technique is present in just a few laboratories all around the world. At the other hand we have generated an easy tool for a quantitative study of pinning energies (model for energy identification at flux line lattice). This tool has been able to work very well at different conditions, giving results for several materials and even for artificially generated vortex systems. Hypothesis considered to the formulation of this model were probed through modeling of virtual systems, supporting the energy model and allowing also defect characterization artificially generated at the YBa2Cu3O7 material. Finally we have carried out an important develop about nature of pinning due to artificial defects, comparing in a quantitative way results associated to different artificial defect generation techniques. This analysis performed in YBa2Cu3O7, is applicable to a broad range of materials as well as topics (e.g. coated conductors, electronic devices, etc.). Here we show a work based on a simple but strong necessity of a clear comparative study between vortex-defect interactions associated to different kind of defects. A work that involves from construction of experimental technique in use to the generation of a tool for the corresponding result analysis, including a final study modeling virtually the systems of vortices experimentally observed. 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