Thesis (MScEng (Electrical and Electronic Engineering))--University of Stellenbosch, 2005. / High-temperature superconductive (HTS) logic has become a large research field worldwide. The HTS advantages compared to the low-temperature circuits are the reduced cooling requirements, increased or comparable operation speeds and the "ease" of manufacturing. The potential of HTS logic circuits is vast and surpassed by very few other technologies. However, before these circuits can reach the market, suitable simulation packages and fabrication processes have to be designed. The simulation of HTS Rapid Single Flux Quantum (RSFQ) circuits are performed at temperatures between 30 and 77 Kelvin. The noise sources in Josephson junctions (JJ’s) are investigated and incorporated into these simulations. The results show that it is possible to operate these circuits in this temperature range. An objective of this thesis is the fabrication of HTS step-edge JJ’s. A process to fabricate these junctions was investigated and improved. This includes the etching of substrate steps with the use of the argon ion mill, deposition of the YBCO thin films with the Pulsed Laser Deposition (PLD) system and the etching of the final circuit by the use of dry or wet etching.

We investigated the electronic structure of the two nematic materials Sr₃Ru₂O₇ and Ca(Fe₀.₉₇Co₀.₀₃As)₂ using spectroscopic – imaging scanning tunneling microscopy (SI-STM) and angle resolved photoemission spectroscopy (ARPES). – – – Sr₃Ru₂O₇ is an itinerant metamagnet that shows a putative quantum critical endpoint at 8 Tesla, submersed by the formation of a nematic electronic phase. Using ARPES, we identified at least 5 Fermi pockets in agreement with quantum oscillation measurements. Surprisingly, we found Fermi velocities up to an order of magnitude lower than in single layer Sr₂RuO₄ and up to 35 times lower than predicted by ab initio calculations. Many bands are confined in an energy range of only ∼10 meV below the Fermi level. This, as well as distinct peak-dip-hump shapes of the spectra with a characteristic energy of around ∼5 meV indicate strong correlations and a possible nontrivial mechanism that is absent in single layer Sr₂RuO₄ and connected to the nematicity. The quasiparticle interference of one of the bands was detected by SI-STM, which was also used to measure subatomic features with the symmetries of the relevant Ru d orbitals. – – – In the second mate- rial, the iron-based high-temperature superconductor Ca(Fe[subscript(1-x)]Co[subscript(x)]As)₂, we discovered electronic nematic nano-pattern in its under-doped ‘parent’ state. We spectroscopically imaged this state in real space over large areas and across domain boundaries that change the directionality of the nano-pattern by 90°. We propose that oriented, dimer-shaped electronic nematogens are responsible for this pattern, in striking contrast to what has been expected and observed in electronic nematic materials. The dimers consist of two Gaussian conductance peaks separated by about 8 a[subscript(FeFe)]. Unidirectionality also shows in the quasiparticle interference pattern of the delocalized electrons. The dispersion is in agreement with scattering from the α₂ band discovered by ARPES but has distinct C₂ symmetry, not inconsistent with a C₄-symmetric band scattered by the proposed dimers.

L'apparition des supraconducteurs à hautes températures (HTc) a permis un développement continu des moteurs supraconducteurs. Ces dispositifs supraconducteurs permettent d'obtenir des puissances et des couples massiques et volumiques très élevés, et sont des alternatives intéressantes aux machines conventionnelles. Le dimensionnement des ces moteurs nécessite une bonne connaissance des caractéristiques électromagnétiques, thermiques et mécaniques des supraconducteurs HTc. Ainsi, le travail présenté ici traite du dimensionnement de machines HTc avec la prise en compte des contraintes liées aux fils supraconducteurs. Le manuscrit débute par une étude bibliographique approfondie dans laquelle trois points sont abordés : les fils supraconducteurs, description des réalisations remarquables de machines HTc de fortes puissances et une analyse statistique sur ces réalisations. Ensuite, la caractérisation DC des fils supraconducteurs HTc et la détermination de leurs pertes AC sous champ variable sont abordées. Des méthodes numériques sont utilisées pour la modélisation et les résultats obtenus sont comparés à ceux de l'expérience. La dernière partie aborde le dimensionnement électromagnétique de moteurs supraconducteurs HTc par des méthodes analytiques et numériques, avec la prise en compte des contraintes liées aux courants critiques dans les bobinages. Les premiers résultats concernant le dimensionnement électromagnétique et thermique d'une structure de machine synchrone supraconductrice à pôles saillants sont donnés. Les solutions techniques envisagées sont largement commentées / The appearance of high-temperature superconductors (HTS) has led to a continuous development of superconducting motors. These superconducting devices allow high power and torque densities, so they constitute and novel alternative to conventional motors. The design of such motors requires a deep knowledge of the electromagnetic, thermal and mechanical characteristics of the HTS materials. Hence, the work presented here is related to the design of HTS motors which includes the constraints due to the HTS conductors. The manuscript begins with an extensive literature review where three main points are discussed, viz. HTS conductors, already constructed HTS motors and statistical analysis on these achievements. The dc characterization of HTS superconductors as well as their ac losses under time varying magnetic fields are theoretically and experimentally studied. In so doing, numerical techniques are used and the obtained results are compared the measurements. The last part deals with the design of HTS motors by analytical and finite element methods. A particular attention is given to the constraints introduced by the use of HTS materials. The first results of the electromagnetic and thermal design of a salient pole HTS synchronous machine are provided with the emphasis of the adopted technical solutions

It is generally believed that one of the key parameterscontrolling the normal state and superconducting properties ofhigh temperature superconductors is the charge carrierconcentrationpin the CuO2planes.By changing the non-isovalent dopingconcentration on the RE site as well as the oxygen content in(RE)Ba2Cu3O7−δ, an excellent tool is obtained tovary the hole concentration over a wide range from theunderdoped up to the overdoped regime.In the present thesis thefocus is on the doping effects on the structural and normalstate electrical properties in Nd-123 doped with Ca, La, Pr,Ca-Pr, and Ca-Th.T he effects of doping have been investigatedby X-ray and neutron powder diffraction, and by measurements ofthe resistivity, thermoelectric powerS, and Hall coefficient RH.T he thermoelectric power is a powerful tool forstudies of high temperature superconductivity and is highlysensitive to details of the electronic band structure.Sas a function of temperature has been analyzed in twodifferent two band models.The parameters of these models arerelated to charactristic features of the electron bands and asemiempirical physical description of the doping dependence ofSis obtained.So me important results are following: (i)The valence of Pr in the RE-123 family.Results from thestructural investigations, the critical temperature Tc, and thethermoelectric power indicated a valence +4 at low dopingconcentration, which is in agreement with results of chargeneutral doping in the RE-123 family.(ii)Hole localization. The results of bond valence sum (BVS)calculations from neutron diffraction data showed that holelocalization on the Pr+4site was the main reason for the decrease of thehole concentration p.Differ ent types of localization wereinferred by S measurements for Ca-Th and Ca-Pr dopings.(iii)Competition between added charge and disorder. Theresults of RH measurements indicated that Ca doping introduceddisorder in the CuO2planes in addition to added charge.This could bethe main reason for the observed small decrease of thebandwidth of the density of states in the description of aphenomenological narrow band model.(iv) Empirical parabolic relation between γ and p.S data were analyzed and well described by a two-band modelwith an additional linear T term, γT.An empiricalparabolic relation for γ as a function of holeconcentration has been found. <b>Key words:</b>high temperature superconductors, criticaltemperature, resistivity, thermoelectric power, Hallcoefficient, X-ray diffraction, Neutron diffraction, NdBa2Cu3O7−δ, hole concentration,substitution.

No description available.

Adaptive microwave systems can benefit from the use of low loss tunable microwave filters. Realizing these tunable filters that show low loss characteristics can be very challenging. The proper materials, tuning elements, and filter designs need to be considered when creating a low loss tunable filter. The integration of low loss microelectromechanical systems (MEMS) and superconducting circuits is one method of achieving these types of tunable filters. The thesis introduces new multi-layer low temperature superconducting (LTS) filters and diplexers and novel topologies for tunable filters and switched multiplexers. An efficient method of designing such filters is proposed. A fabrication process to monolithically integrate MEMS devices with high temperature superconducting (HTS) circuits is also investigated in this thesis. The reflected group delay method, usually used for filter tuning, is further developed for use in designing microwave filters. It is advantageous in the design of filters to have electromagnetic simulation results that will correlate well to the fabricated microwave filters. A correction factor is presented for use with the reflected group delay method so the group delay needs to be matched to the appropriate value at the center frequency of the filter and be symmetric about the center frequency of the filter. As demonstrated with an ideal lumped element filter, the group delay method can be implemented when a closed form expression for the circuit is not known. An 8-pole HTS filter design and an 8-pole multi-layer LTS filter design demonstrate the use of the reflected group delay method. Low temperature superconducting filters, couplers and diplexers are designed and fabricated using a multilayer niobium fabrication process traditionally used for superconducting digital microelectronics. The feasibility of realizing highly miniaturized microwave niobium devices allows for the integration of superconducting digital microelectronics circuits and analog microwave devices on a single chip. Microwave devices such as bandpass filters, lowpass filters, bandstop filters, quadrature hybrids, and resistive loads are all demonstrated experimentally. New tunable filter designs are presented that can make use of MEMS switches. A manifold-coupled switched multiplexer that allows for 2^N possible states is presented. The tunable multiplexer has N filters connected to two manifolds and has embedded switches, which detune certain resonators within the filters to switch between ON and OFF states for each channel. The new concept is demonstrated with a diplexer design and two 3-pole coplanar filters. The concept is further developed through test results of a fabricated HTS triplexer and electromagnetic simulations to demonstrate a superconducting manifold-coupled switched triplexer. Another filter design is presented that makes use of switches placed only on the resonators of the filters. This filter design has N possible states and the absolute bandwidth can be kept constant for all N states. Finally, the integration of HTS circuits and MEMS devices is investigated to realize low loss tunable microwave filters. The hybrid integration is first performed through the integration of an HTS microstrip filter and commercially available RF MEMS switches. A fabrication process to monolithically integrate MEMS devices and high temperature superconducting circuits is then investigated. The fabrication process includes a titanium tungsten layer, which acts as both a resistive layer and an adhesion for the dielectric layer, an amorphous silicon dielectric layer, a photoresist sacrificial layer, and the top gold layer. The fabrication process is built up on a wafer with a thin film of a high temperature superconducting material covered with a thin film of gold. Several processes are tested to ensure that the superconducting properties of the thin film are not affected during the MEMS fabrication process.

<p>It is generally believed that one of the key parameterscontrolling the normal state and superconducting properties ofhigh temperature superconductors is the charge carrierconcentration<i>p</i>in the CuO₂planes.By changing the non-isovalent dopingconcentration on the RE site as well as the oxygen content in(RE)Ba₂Cu₃O_7−δ, an excellent tool is obtained tovary the hole concentration over a wide range from theunderdoped up to the overdoped regime.In the present thesis thefocus is on the doping effects on the structural and normalstate electrical properties in Nd-123 doped with Ca, La, Pr,Ca-Pr, and Ca-Th.T he effects of doping have been investigatedby X-ray and neutron powder diffraction, and by measurements ofthe resistivity, thermoelectric power<i>S</i>, and Hall coefficient R_H.T he thermoelectric power is a powerful tool forstudies of high temperature superconductivity and is highlysensitive to details of the electronic band structure.<i>S</i>as a function of temperature has been analyzed in twodifferent two band models.The parameters of these models arerelated to charactristic features of the electron bands and asemiempirical physical description of the doping dependence of<i>S</i>is obtained.So me important results are following:</p><p>(i)<i>The valence of Pr in the RE-123 family.</i>Results from thestructural investigations, the critical temperature Tc, and thethermoelectric power indicated a valence +4 at low dopingconcentration, which is in agreement with results of chargeneutral doping in the RE-123 family.(ii)<i>Hole localization</i>. The results of bond valence sum (BVS)calculations from neutron diffraction data showed that holelocalization on the Pr⁺⁴site was the main reason for the decrease of thehole concentration p.Differ ent types of localization wereinferred by S measurements for Ca-Th and Ca-Pr dopings.(iii)<i>Competition between added charge and disorder</i>. Theresults of RH measurements indicated that Ca doping introduceddisorder in the CuO₂planes in addition to added charge.This could bethe main reason for the observed small decrease of thebandwidth of the density of states in the description of aphenomenological narrow band model.(iv) Empirical parabolic relation between γ and p.S data were analyzed and well described by a two-band modelwith an additional linear T term, γT.An empiricalparabolic relation for γ as a function of holeconcentration has been found.</p><p><b>Key words:</b>high temperature superconductors, criticaltemperature, resistivity, thermoelectric power, Hallcoefficient, X-ray diffraction, Neutron diffraction, NdBa₂Cu₃O_7−δ, hole concentration,substitution.</p>

Adaptive microwave systems can benefit from the use of low loss tunable microwave filters. Realizing these tunable filters that show low loss characteristics can be very challenging. The proper materials, tuning elements, and filter designs need to be considered when creating a low loss tunable filter. The integration of low loss microelectromechanical systems (MEMS) and superconducting circuits is one method of achieving these types of tunable filters. The thesis introduces new multi-layer low temperature superconducting (LTS) filters and diplexers and novel topologies for tunable filters and switched multiplexers. An efficient method of designing such filters is proposed. A fabrication process to monolithically integrate MEMS devices with high temperature superconducting (HTS) circuits is also investigated in this thesis. The reflected group delay method, usually used for filter tuning, is further developed for use in designing microwave filters. It is advantageous in the design of filters to have electromagnetic simulation results that will correlate well to the fabricated microwave filters. A correction factor is presented for use with the reflected group delay method so the group delay needs to be matched to the appropriate value at the center frequency of the filter and be symmetric about the center frequency of the filter. As demonstrated with an ideal lumped element filter, the group delay method can be implemented when a closed form expression for the circuit is not known. An 8-pole HTS filter design and an 8-pole multi-layer LTS filter design demonstrate the use of the reflected group delay method. Low temperature superconducting filters, couplers and diplexers are designed and fabricated using a multilayer niobium fabrication process traditionally used for superconducting digital microelectronics. The feasibility of realizing highly miniaturized microwave niobium devices allows for the integration of superconducting digital microelectronics circuits and analog microwave devices on a single chip. Microwave devices such as bandpass filters, lowpass filters, bandstop filters, quadrature hybrids, and resistive loads are all demonstrated experimentally. New tunable filter designs are presented that can make use of MEMS switches. A manifold-coupled switched multiplexer that allows for 2^N possible states is presented. The tunable multiplexer has N filters connected to two manifolds and has embedded switches, which detune certain resonators within the filters to switch between ON and OFF states for each channel. The new concept is demonstrated with a diplexer design and two 3-pole coplanar filters. The concept is further developed through test results of a fabricated HTS triplexer and electromagnetic simulations to demonstrate a superconducting manifold-coupled switched triplexer. Another filter design is presented that makes use of switches placed only on the resonators of the filters. This filter design has N possible states and the absolute bandwidth can be kept constant for all N states. Finally, the integration of HTS circuits and MEMS devices is investigated to realize low loss tunable microwave filters. The hybrid integration is first performed through the integration of an HTS microstrip filter and commercially available RF MEMS switches. A fabrication process to monolithically integrate MEMS devices and high temperature superconducting circuits is then investigated. The fabrication process includes a titanium tungsten layer, which acts as both a resistive layer and an adhesion for the dielectric layer, an amorphous silicon dielectric layer, a photoresist sacrificial layer, and the top gold layer. The fabrication process is built up on a wafer with a thin film of a high temperature superconducting material covered with a thin film of gold. Several processes are tested to ensure that the superconducting properties of the thin film are not affected during the MEMS fabrication process.

Thesis (Ph. D.)--University of Washington, 1998. / Vita. Includes bibliographical references (p. [225]-236).

Les récents progrès des supraconducteurs à haute température critique (SHTC) rendent ces matériaux très prometteurs pour les applications de grande envergure. Ils montrent des propriétés stupéfiantes, particulièrement à très basse température où ils sont capables de transporter des courants extrêmement élevés. Ces conducteurs sont construits sur un substrat en alliage de nickel qui leurs permettent de supporter des efforts mécaniques considérables. Pour ces raisons, les SHTC sont à présent au cœur de nombreux projets pour bâtir une nouvelle génération d’aimants produisant des champs intenses. Ce travail est mené dans le cadre du projet Nougat dont l’objectif est la conception d’un insert générant 10 T à l’intérieur d’un aimant plus grand de 20 T. Malgré leurs remarquables propriétés, les conducteurs SHTC montrent d’importantes inhomogénéités de performance ce qui, ajouté à leur forte capacité calorifique, peut provoquer des échauffements très localisés. La transition locale du matériau vers un état dissipatif est difficile à détecter notamment à cause de la lente propagation de l’évènement. Le signal de transition risque donc d’être noyé dans le bruit de mesure et être à l’origine de dommages sévères voir irréversibles du conducteur. Pour répondre à ce problème, des électro-aimants sont fabriqués en remplaçant l’isolation électrique par un ruban métallique. Ces aimants sont appelées Bobines Métal-Isolée (BMI). Dans cette configuration et dans le cas spécifique d’une inhomogénéité sur le conducteur, le courant court-circuite le défaut en passant sur la spire suivante ce qui permet d’empêcher la destruction du bobinage. Cependant, dans ce type de bobinage, la stabilité et la linéarité du champ magnétique généré peut être un problème et les dynamiques lentes de ce type de bobine les empêchent de prétendre à être utilisé pour certaines applications. Pour des raisons pratiques, le prototype final du projet Nougat sera construit avec un bobinage métalliquement isolé mais le travail mené ici se concentre sur les bobines électriquement isolées, plus particulièrement l’étude de la transition dans le bobinage afin d’identifier des possibilités de protection fiables. Quelques résultats d’échantillons BMI sont cependant présentés pour comparaison. Dans un premier temps, les contraintes générales de conception de l’insert NOUGAT sont définies, principalement les exigences pour le conducteur et les calculs préliminaires de performance. Cette étude souligne le besoin de caractériser le conducteur SHTC sous haut champ et à basse température, dans des conditions similaires à celles du fonctionnement final du prototype d’insert. Dans un second temps, cette caractérisation expérimentale d’échantillons courts est implémentée dans un modèle pour simuler le comportement de bobines pour la construction d’aimants. L’objectif de ce modèle est d’étudier le départ de transition et sa propagation dans le bobinage. Une des particularités du modèle est la simulation de l’entièreté du bobinage dans le but de prendre en compte les inhomogénéités de performance du conducteur, dont les mesures en continu sont fournies à 77 K en champ propre par les constructeurs. Le dernier axe de ce travail est l’étude de plusieurs échantillons bobinés pour permettre notamment la comparaison entre bobinage isolé électriquement et métalliquement. Leurs comportements électrique, mécanique et thermique sont examinés ainsi que les problèmes de couplage dus à la présence d’un aimant extérieur. Les résultats expérimentaux sont comparés aux calculs préliminaires ainsi qu’aux simulations de modèle. Un protocole expérimental est également proposé pour évaluer les performances d’une bobine sans risque et est testé avec succès. / Recent improvements in High Temperature Superconductors (HTS) make them promising for large scale applications. They show astonishing properties, especially at very low temperature where they are able to carry high amount of current. These conductors are also built on a nickel alloy substrate allowing them to face severe mechanical stresses. For these reasons, HTS are now placed at the heart of numerous projects for building a next generation of high field magnets. This work is conducted in the NOUGAT project, which intends to design and build a 10 T HTS insert working in a 20 T background field. Despite their outstanding properties, HTS conductors show strong inhomogeneities in their performance along their length. This added to their high heat capacity can be at the origin of local hot spots. The transition to a dissipative state on this local area is then difficult to detect; because of the low speed of its propagation. The transition signal is likely to be lost in the high noise level environment, which can lead the winding to severe or irreversible damages.One way of dealing with this problem is to create coils where the electric insulation is removed and replaced by a metallic layer, the so-called Metal-as-Insulation winding technique. In this configuration and in a case of an inhomogeneity, the current bypasses through the turn-to-turn contact resistance and prevents the winding from burning. However, in such kind of winding field stability and linearity can be an issue. The slower dynamics obtained with this method prevent its use in some applications.The main focus of this work is therefore insulated coils especially the study of the transition behaviour to explore the possibility of reliable protections. In the same extend, the present work also considers safe ways to evaluate the performance of a wound sample in order for it to work with appropriate margins when at nominal operating conditions. This would decrease the need of a strong detection/protection system: the high heat capacity of the conductor makes windings unlikely to be damaged by an outside event.For practical reasons, the NOUGAT project will be built using MI technique, and therefore some MI coils results are presented in this work for comparison purpose. In a first step, the general design constrains for the NOUGAT project insert are defined, especially tape requirements and performance calculations. This underlines the necessity of characterizing the HTS conductor under high field and at low temperature, under conditions similar to the expected operation of the final insert prototype.In a second step, this experimental short sample characterization is implemented in a model to simulate the behaviour of pancake coils designed to build magnet. The aim of the model is to investigate on the transition start and propagation inside the winding. One of the main specificities is the simulation of the whole winding taking as an input the continuous critical current density measurement given by the providers at 77 K, self-field.The last axis of this work is the study of several wound samples allowing comparison between both insulated and metallic insulated windings. Their electric, magnetic, mechanic and thermal behaviours are examined as well as the coupling issues while working inside an outer magnet. Experimental results are compared to preliminary calculations and modelling results. An experimental protocol to safely evaluate coil performance margins is also proposed and tested successfully.Conclusions are then drawn about the possibility to operate safely full scale HTS magnets with present day conductor performances.