Publication: Efectos mesoscópicos y longitudes características en sistemas superconductores
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2016-02-18
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Universidad Complutense de Madrid
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Abstract
En esta tesis se han estudiado varios efectos en el estado mixto de superconductores, cuando están en el régimen mesoscópico. Es decir, cuando el superconductor esta nanoestructurado con tamaños comparables a las longitudes físicas características del superconductor: la longitud coherente y la longitud de penetración. Se han utilizado dos tipos de superconductores y dos modos de introducir la nanoestructuración. Se han utilizado monocristales superconductores laminares, que están nanoestructurados en sí mismos; y películas delgadas superconductoras, en las que se han introducido artificialmente patrones de defectos no superconductores de los tamaños adecuados para poder tener efectos mesoscópicos. Para esto último se han crecido y nanoestructurado películas superconductoras mediante diversas técnicas de micro y nanofabricación (como pulverización catódica, litografía por haz de electrones, litografía óptica o ataque por iones reactivos) con el fin de generar potenciales artificiales de anclaje de vórtices. De esta manera se han estudiado diversos fenómenos de competencia entre superconductividad, magnetismo, geometría y longitudes características. Se ha estudiado la dinámica de vórtices mediante medidas de transporte eléctrico a bajas temperaturas y altos campos magnéticos. Para ello se ha usado un criostato de helio líquido de flujo continuo que permite variar la temperatura de la muestra entre 1.3 K y 300 K, con una bobina superconductora con capacidad de aplicar un campo magnético de 9T; un sistema de recuperación y licuefacción del helio; y un licuefactor de nitrógeno..
In this thesis, different effects have been studied in the mixed state of superconductors, when they are in a mesoscopic regime. That is, when the superconductor is nanostructured with typical sizes in the same scale that the characteristic lengths of the superconductor: the coherence and the penetration lengths. Two kind of superconductors, with two different ways to introduce nanostructuration have been used: layered superconducting monocrystals, which are nanostructured by their structure; and superconducting thin films, in which artificially generated non-superconducting defects have been embedded, with the proper size to generate mesoscopic effects. Fort the last, superconducting thin films have been grown, and have been nanostructured by means of different micro and nanofabrication techniques (such as magnetron sputtering, electron beam lithography or reactive ion etching), to generate artificial vortex pinning potentials. This way, different phenomena derived from the competition of superconductivity, magnetism, geometry and characteristic lengths have been studied. Vortex dynamics have been studied using electric transport measurements at low temperatures and high magnetic fields. For this purpose, a continuous flow liquid helium cryostat (which allows varying the temperature between 1.3 K and 300 K and has a superconducting solenoid capable of generate a 9T magnetic field), a helium recovery and liquefying system and a nitrogen liquefier have been used...
In this thesis, different effects have been studied in the mixed state of superconductors, when they are in a mesoscopic regime. That is, when the superconductor is nanostructured with typical sizes in the same scale that the characteristic lengths of the superconductor: the coherence and the penetration lengths. Two kind of superconductors, with two different ways to introduce nanostructuration have been used: layered superconducting monocrystals, which are nanostructured by their structure; and superconducting thin films, in which artificially generated non-superconducting defects have been embedded, with the proper size to generate mesoscopic effects. Fort the last, superconducting thin films have been grown, and have been nanostructured by means of different micro and nanofabrication techniques (such as magnetron sputtering, electron beam lithography or reactive ion etching), to generate artificial vortex pinning potentials. This way, different phenomena derived from the competition of superconductivity, magnetism, geometry and characteristic lengths have been studied. Vortex dynamics have been studied using electric transport measurements at low temperatures and high magnetic fields. For this purpose, a continuous flow liquid helium cryostat (which allows varying the temperature between 1.3 K and 300 K and has a superconducting solenoid capable of generate a 9T magnetic field), a helium recovery and liquefying system and a nitrogen liquefier have been used...
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física de Materiales, leída el 18-12-2015