Pinning landscapes in modern second generation coated conductors are excellent candidates for studies of vortex pinning. The ability to produce engineered defects in thin films of high temperature superconductors allows one to investigate representative distinct pinning sites, with the objective of understanding how different pinning centers contribute, compete and evolve under varying conditions of magnetic field strength and orientation, and temperature.New contact-free methods were developed specifically to investigate this system in new ways, especially the dependence of the critical current density Jc on orientation of the magnetic field. A superconducting quantum interference device (SQUID)-based magnetometer was used to determine angular critical current density profiles. The induced currents produced in contact-free methods allow one to investigate a range of temperatures that is difficult to access by traditional transport measurements.Materials with three distinctive pinning landscapes were investigated: Specifically, samples were studied that were dominated by columnar defects, by isotropic defects, or a mixture of these two types of pins. These studies over an expanded temperature-field-orientation phase-space clearly show competition between not only strong and weak pinning centers, but also between the angularly selective and nonselective pinning. This evidence is seen in critical current density profiles, the α [alpha]-values describing the falloff of Jc with magnetic field, and matching field effects in the three samples studied.

High-quality platelet single crystals of Tl₂Ba₂CuO₆±δ (Tl-2201) have been grown using a novel time-varying encapsulation scheme, minimizing the thallium oxide loss that has plagued other attempts and reducing cation substitution. This encapsulation scheme allows the melt to be decanted from the crystals, a step previously impossible, and the remaining cation substitution is homogenized via a high-temperature anneal. Oxygen annealing schemes were developed to produce sharp superconducting transitions from 5 to 85 K without damaging the crystals. The crystals' high homogeneity and high degree of crystalline perfection are further evidenced by narrow rocking curves; the crystals are comparable to YSZ-grown YBa₂Cu₃O₆₊δ by both metrics. Electron probe microanalysis (EPMA) ascertained the crystals' composition to be Tl₁.₉₂₀₍₂₎Ba₁.₉₆₍₂₎Cu₁.₀₈₀₍₂₎O₆₊δ; X-ray diffraction found the composition of a Tc = 75 K crystal to be Tl₁.₉₁₄₍₁₄₎Ba₂Cu₁.₀₈₆₍₁₄₎O₆.₀₇₍₅₎, in excellent agreement. X-ray refinement of the crystal structure found the crystals orthorhombic at most dopings, and their structure to be in general agreement with previous powder data. Cation-substituted Tl-2201 can be orthorhombic, orthorhombic crystals can be prepared, and these superconduct, all new results. X-ray diffraction also found evidence of an as yet unidentified commensurate superlattice modulation. The Tl-2201 crystals' electronic structure were studied by X-ray absorption and emission spectroscopies (XAS/XES). The Zhang-Rice singlet band gains less intensity on overdoping than expected, suggesting a breakdown of the Zhang-Rice singlet approximation, and one thallium oxide band does not disperse as expected. The spectra correspond very closely with LDA band structure calculations, and do not exhibit the upper Hubbard bands arising from strong correlations seen in other cuprates. The spectra are noteworthy for their unprecedented (in the high-Tc cuprates) simplicity. The startling degree to which the electronic structure can be explained bodes well for future research in the cuprates. The overdoped cuprates, and Tl-2201 in particular, may offer a unique opportunity for understanding in an otherwise highly confusing family of materials.

Thesis (Ph.D.) -- McMaster University, 1999. / Disk in pocket. Includes bibliographical references (leaves 232-237). Also available via World Wide Web.

Thesis (Ph.D.)--University of Wollongong, 2005. / Typescript. Includes bibliographical references: leaf 188-200.

Thesis (Ph.D.); Submitted to Iowa State Univ., Ames, IA (US); 12 Dec 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "IS-T 2605" Chang-Hwan Kim. 12/12/2003. Report is also available in paper and microfiche from NTIS.

Thesis (M.S.)--Florida State University, 2006. / Advisor: Justin Schwartz, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 19, 2006). Document formatted into pages; contains xii, 43 pages. Includes bibliographical references.

Thesis (MScEng)--University of Stellenbosch, 2002. / ENGLISH ABSTRACT: High-temperature (HTc) SQUID magnetometers are used in the biomedical field of magnetocardiography (MCG). The use of MCG enables a much earlier diagnosis of potentially dangerous conditions than the use of electrocardiography (EeG). For the use of MCG, high-quality HTc SQUIDs are necessary. These SQUIDs can only be realised if the design and fabrication processes are well defined and understood. In this thesis an extensive literature review on the design process explains all the concepts necessary to understand and design a high-quality HTc SQUID. Then the fabrication process is redefined and characterised. The SQUIDs are fabricated with YBCO thin films on MgO substrates. The relevant steps in the process are the pulsed laser deposition (PLD), UV lithography and argon ion milling. The characterisation of the fabrication process is based on a statistical method called Design of Experiment (DOE). This enables more information to be obtained about the fabrication process than before. For the PLO process the important parameters are determined and an initial optimisation is done. During the characterisation process it is proven that the use of ozone during PLO has a positive effect on the quality of the YBCO thin film. On the PLD apparatus the substrate heater and laser-beam profile is improved. The UV lithography process is analysed and a DOE is proposed for optimisation. A temperature bath is added to the process to enable better reproducibility. An rf multicusp ion mill is built for argon-ion milling, this device is tested and shown to provide the required etching capability. / AFRIKAANSE OPSOMMING: Hoë-temperatuur (HTc) SQUID magnetometers word in die biomediese rigting vir magnetokardiografie (MKG) gebruik. Met MKG kan 'n mens potensieel gevaarlike toestande heelwat vroeër diagnoseer as wat moontlik is met elektrokardiografie (EKG). HTc SQUIDs van hoë kwaliteit word benodig vir MKG. Sulke SQUIDs kan slegs vervaardig word as die vervaardiging en ontwerpsprosesse goed gedefinieer en verstaan word. In hierdie tesis verduidelik 'n omvattende literatuurstudie oor die ontwerpsproses al die konsepte wat nodig is om 'n SQUID van hoë kwaliteit te verstaan en te ontwerp. Verder word die vervaardigingsproses geherdefinieer en gekarakteriseer. Die SQUIDs word vervaardig met YBCO-dunfilm op MgO-substrate. Die relevante stappe tydens die vervaardiging is: gepulseerde laser deposisie (PLO), UV-litografie en argon-ioonstraling. Die karakterisering van die vervaardigingsproses is gebaseer op 'n statistiese metode genaamd "Design of Experiment" (DOE). Hierdie metode laat 'n mens toe om meer inligting oor die proses te verkry as wat voorheen moontlik was. Gedurende die karakterisering word bewys dat die gebruik van osoon gedurende die PLO-proses 'n positiewe invloed op die kwaliteit van die YBCO-dunfilm het. Die PLO-apparaat se substraatverwarmer en laserstraalprofiel word verbeter. Die UV-litografieproses word geanaliseer en 'n DOE word voorgestel vir die optimisering daarvan. Daar word 'n temperatuurbad by die proses gevoeg om die herhaalbaarheid te verbeter. 'n "Rf multicusp ion mill" word gebou, die toestel word getoets en daar word aangetoon dat dit die nodige etsvermoë het.

Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: High Temperature Superconductors (HTS) exhibit exceptional electrical properties that make them attractive candidates for numerous electronic devices and applications. However, constructing working devices can be challenging due to fabrication difficulties of these brittle ceramics. This thesis investigates new methods to make the fabrication of high quality YBa2Cu3O7 (YBCO) thin films easier and compatible with more materials. We present the development of a universal add-on method that can be used in situ to improve the quality of superconducting thin films deposited by Pulsed Laser deposition (PLD). We investigate the in situ application of electric fields and voltage biasing to improve the thin film growth. Considering various electrode configurations, we have developed a final electrode design that is stable and produces reproducible results. By introducing an insulated high voltage (HV) electrode into the chamber during deposition, the quality of the deposited thin films can be modulated depending on the polarity of the voltage applied. Applying a positive voltage improves the film quality obtained. Applying a negative voltage degrades the superconducting properties of the films. A simple proof-of-concept HTS dual-mode microwave filter was designed, fabricated and tested. Only the filter produced using our novel PLD technique displayed the correct filtering action upon cooling to 77K. This is attributed to the thin films better superconducting properties due to our developed technique. / AFRIKAANSE OPSOMMING: Hoë Temperatuur Supergeleiers (HTS) vertoon aantreklike elektriese eienskappe wat hulle goeie kandidate maak vir verskeie elektroniese toepassings. Om werkende toestelle te ontwikkel kan 'n uitdaging wees, as gevolg van die vervaardigings probleme wat bestaan vir hierdie bros keramiek materiaal. Hierdie tesis ondersoek nuwe metodes om die vervaardiging van 'n hoë gehalte YBa2Cu3O7 (YBCO) dun films makliker en versoenbaar te maak met verskeie materiale. Ons toon die ontwikkeling van 'n algemene metode wat maklik bygevoeg kan word om in situ die gehalte van supergeleidende dun films, wat deur gepulseerde laser deponering (PLD) gedeponeer is, te verbeter. Ons ondersoek die in situ toepassing van elektriese velde en spannings om die dun film groei te verbeter. Verder oorweeg ons verskeie elektrode konfigurasies en ontwikkel 'n finale elektrode ontwerp wat stabiel is en herhaalbare resultate produseer. Die kwaliteit van die gedeponeerde dun films kan gemoduleer word deur die byvoeging van 'n geïsoleerde hoogspannings (HV) elektrode tydens deponering, afhangende van die polariteit van die aangelegde spanning. 'n Positiewe spanning verhoog die film kwaliteit, terwyl 'n negatiewe spanning die supergeleidende eienskappe van die films verlaag. 'n Eenvoudige HTS dubbele-modus mikrogolffilter is ontwerp, vervaardig en getoets, om as toepassings voorbeeld te dien. Slegs die filter wat geproduseer was met behulp van ons nuwe PLD tegniek, vertoon die beste filter oordrag by 77K. Dit word toegeskryf aan die beter supergeleidende eienskappe van die dun film, as gevolg van die toepassing van ons ontwikkelde tegniek.

Made available in DSpace on 2014-06-11T19:23:29Z (GMT). No. of bitstreams: 0 Previous issue date: 2006-06-30Bitstream added on 2014-06-13T19:50:18Z : No. of bitstreams: 1 nascimento_rm_me_bauru.pdf: 3217433 bytes, checksum: 4db978f65eff59d8b027881dac872d0f (MD5) / Secretaria de Educação do Estado de São Paulo / Desde a descoberta dos supercondutores com alta Tc, vários trabalhos têm sido efetuados sobre as diferentes propriedades destes materiais. As cerâmicas Y'BA IND.2' C IND.U 'IND.3' 'O IND. 7-'delta' mostraram que tem suas propriedades supercondutoras fortemente afetadas pela quantidade de oxigênio. Mais recentemente, medidas de relaxações anelásticas em 'LA IND.2'CU 'O IND. 4+'delta' mostraram que a remoção deste elemento pode ser relacionada com dois eventos. Um deles é o decréscimo na mobilidade entre dois planos adjacentes CuO e o outro, é o aumento no número de padrões de mobilidade para os octaedros CU 'O IND. 6'. Para SM'BA IND.2' C IND.U 'IND.3' 'O IND. 7-'delta' (SBCO), a possibilidade de uma estequiometria de oxigênio variável e sua alta mobilidade nos planos... / Since the discovery of the high Tc superconductors, several works have been made about the different propoerties of these materials. Y'BA IND.2' C IND.U 'IND.3' 'O IND. 7-'delta' ceramic showed to have its superconducting properties strongly affected by the oxygen content. More recently, anelastic relaxation measurements in 'LA IND.2'CU 'O IND. 4+'delta', showed that the remotion of this element can be related to two events. One is the decrease in mobility between two adjacent CuO planes, and the other is the increase in the number of tilting patterns of the Cu 'O IND. 6' octahedra. For SM'BA IND.2' C IND.U 'IND.3' 'O IND. 7-'delta' (SBCO), the possibility of variable stoichiometry and the high mobility of oxygen in the... (Complete abstract, click electronic access below)

Nós estudamos a competição entre a instabilidade de Pomeranchuk no canal de spin com momento angular l=1 e uma interação atrativa, favorecendo a formação de um par de Cooper. Achamos, numa aproximação de campo médio, uma forte supressão da instabilidade de Pomeranchuk via supercondutividade. Além disso, identificamos uma fase supercondutora metaestável com características semelhantes ao estado FFLO. Um líquido de Fermi é, com exceção de uma dimensão, um estado muito estável da matéria. Por outro lado dois tipos de instabilidades, relacionadas com interações atrativas, são conhecidas: Instabilidades Pomeranchuk e supercondutora. As instabilidades Pomeranchuk ocorrem na presença da interação de dois corpos contendo uma forte componente atrativa no canal de espalhamento para frente com momento angular definido. No contexto da teoria de Landau, a instabilidade ocorre quando um ou mais parâmetros admensionais de Landau nos canais de spin ou carga, adquirem altos valores negativos. As instabilidades Pomeranchuk no setor de carga quebram a simetria de rotação. Em particular, uma instabilidade em alguns canais produz uma deformação elipsoidal na superfície de Fermi. / We study the competition between a Pomeranchuk instability in the spin channel with angular momentum l = 1 and an attractive interaction, favoring Cooper pair formation. We find, at mean-field approximation, that superconductivity strongly suppress the Pomeranchuk instability. Moreover, we have found a metastable modulated superconducting phase with similar characteristics of the FFLO state. A Fermi liquid is, except in one dimension, a very stable state of matter. At least two types of instabilities, related with attractive interactions, are known: Pomeranchuk and superconducting instabilities. Pomeranchuk instabilities occur in the presence of two-body interactions containing a strong attractive component in the forward scattering channel with a definite angular momentum. In the context of Landau theory, the instability sets in when one or more dimensionless Landau parameters in the charge or spin channel, acquire large negative values. Pomeranchuk instabilities in a charge sector spontaneously break rotational symmetry. In particular, an instability in the some channels produces an ellipsoidal deformation of the Fermi surface.