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In der vorliegenden Arbeit wurden Schichten des Hochtemperatursupraleiters YBa2Cu3O7-δ (YBCO) untersucht, die epitaktisch mittels gepulster Laserabscheidung auf texturierten metallischen Templaten abgeschieden wurden. Dabei kamen ionenstrahltexturiertes ABAD-YSZ-Band und walztexturiertes NiW-Band zum Einsatz. Für Anwendungen solcher sogenannten Bandleiter ist die Fähigkeit zum Transport möglichst hoher kritischer Ströme Ic essentiell. Dies kann durch das Wachstum möglichst dicker Schichten, eine Verbesserung der Flussschlauchverankerung (Pinning), sowie die Reduzierung der durch Korngrenzen verursachten Stromlimitierung erreicht werden. Im ersten Teil der Arbeit wurden strukturelle und supraleitende Eigenschaften in bis zu 5 µm dicken YBCO-Schichten auf ABAD-YSZ-Band untersucht. Dazu wurden neben nicht-fremdphasendotierten (undotierten) YBCO-Schichten auch BaHfO3- und BaY(NbTa)O6-dotierte YBCO-Schichten herangezogen. Die Untersuchungen erfolgten mittels Röntgenbeugungs-methoden (XRD), Rasterelektronen- und Rasterkraftmikroskopie (REM, AFM), sowie resistiver und induktiver Methoden zur Bestimmung der Sprungtemperatur Tc und der kritischen Stromdichte Jc. Schichtdickenabhängige Messungen konnten u. a. zeigen, dass Ic in undotierten YBCO-Schichten bei einer Dicke oberhalb von 2,8 µm nicht weiter anstieg, während Ic in den dotierten Schichten bis zu einer Dicke von 5 µm nicht limitiert war. Darüber hinaus konnte in temperatur- und feldabhängigen Jc-Messungen ein verbessertes Pinningverhalten bei tiefen Temperaturen und in hohen äußeren Magnetfeldern gefunden werden. Mit Hilfe von transmissionselektronenmikroskopischen Untersuchungen (TEM) und Messungen der Jc-Anisotropie wurde zusätzlich ein Strukturschema herausgearbeitet, das eine Verknüpfung von Herstellungsparametern, Mikrostruktur und Pinningeigenschaften ermöglicht. Der zweite Teil befasste sich mit der Analyse der Mikrostruktur und der lokalen Textur von undotierten YBCO-Schichten auf ABAD-YSZ- und NiW-Band. Die lokale Textur wurde dabei mittels Rückstreuelektronenbeugung (EBSD) aufgelöst. Die Ergebnisse wurden mit orts-aufgelösten Magnetisierungsmessungen (Raster-Hall-Sonden-Mikroskopie) korreliert, mit denen der lokale Stromfluss untersucht werden kann. Auf beiden Templaten zeigte sich eine granulare Struktur, die durch gegeneinander verkippte bzw. verdrehte YBCO-Körner gekennzeichnet war. Während die granulare Struktur auf ABAD-YSZ-Band durch hauptsächlich in-plane missorientierte YBCO-Bereiche mit einem Durchmesser < 1 µm charakterisiert war, konnten auf NiW-Band hauptsächlich out-of-plane missorientierte Bereiche mit einer typischen Größe von 20-50 µm nachgewiesen werden. Auf NiW-Band hing die YBCO-Mikrostruktur und die Schärfe der lokalen Textur von der individuellen out-of-plane Missorientierung des darunter-liegenden NiW-Korns ab, was auf eine bereits im unbeschichteten NiW-Band vorhandene Facettierung zurückgeführt werden konnte. Zu deren Beschreibung wurde ein Facettenmodel entwickelt, das durch TEM-Untersuchungen bestätigt werden konnte. Abschließend wurde auf einkristallinen Substraten und auf ABAD-YSZ-Band ein homogener Stromfluss nachgewiesen, während die supraleitenden Eigenschaften in YBCO-Schichten auf NiW-Band von der lokalen Mikrostruktur und Textur bestimmt wurden. Damit konnte gezeigt werden, dass die globalen Eigenschaften der Bandleiter vom Zusammenspiel eines über Korngrenzen miteinander verbundenen Ensembles von Körnern mit individuellen Eigenschaften bestimmt werden.

While in conventional superconductors coupling between electrons and phonons is known to be responsible for the electron pairing, for the high temperature superconductors the pairing media remains under debates. Since the interactions of electrons with other degrees of freedom (phonons, magnetic excitations, etc) manifest themselves by an additional renormalization in the electronic dispersion, they can be investigated by means of Angle Resolved Photoelectron Spectroscopy. In the work renormalization in two families of high Tc cuprates have been studied. Along the diagonal of the two-dimensional BZ, the renormalization effects are represented by an unusual band dispersion that develops a so-called ‘‘kink’’. In the vicinity of the (pi, 0) point of the BZ, where the order parameter reaches its maximum, the renormalization is noticeably stronger and makes itself evident even in the shape of a single spectral line measured for a fixed momentum. It was shown that for the Bi-2212 samples substitution of Cu atoms in Cu-O plane changes renormalization features in ARPES spectra both in nodal and antinodal parts of the Brillouin zone. The smearing of the dip in the in the spectral line shape measured at (pi; 0) point can be well explained by coupling of electrons to the magnetic resonance mode. The effect of Zn and Ni substitution on the antinodal ARPES spectra was shown to be in good agreement with the influence of these impurities on magnetic resonance mode seen in inelastic neutron scattering experiments. This, in addition to the previous ARPES studies of temperature and doping dependence of peak-dip-hump structure, mass renormalization near antinodal region and a kink in the nodal part of Brillouin zone, provides further evidence that the coupling to magnetic excitations, rather than to phonons, is responsible for the observed unusual renormalization. Unlike the well studied Bi-2212 family of cuprates, photoemission on YBCO-123 turns out to be much more complicated. The observed spectra have a strong contribution from a heavily overdoped surface component with the hole doping level of about x~0.30, which is weakly dependent on the sample stochiometry. Absence of any signs of superconductivity in the spectra of the overdoped component was argued to result from the unusually high doping level. This conclusion is supported by the fact that the overdoped bands give rise to the Fermi surface and band structure consistent with the predictions of the LDA calculations, as well as, by the dependence of the photoemission matrix element on the excitation energy, which closely follows that of the superconducting bulk component. Specific experimental geometry was used to enhance the signal coming from the superconducting component. In particular, experiments with circularly polarized light bundled with simple theoretical considerations enabled better separation of the surface and the bulk components. This type of experiments also suggests that the overdoped component is mainly localized in the topmost CuO2 bilayer, while the next bilayers in the YBCO-123 structure already represent bulk properties and retain superconductivity. Using partially Ca substituted samples it was possible to obtain spectra with a suppressed overdoped component. The likely reason for the suppression is a shift of the most probable cleavage plane from the Ba–O interface to the Y layer. Spectra from the Ca substituted sample clearly reveal a sizable superconducting gap, and strong renormalization effects in the vicinity of the antinodal point. The fact that the renormalization vanishes above Tc and has strong momentum dependence, diminishing away from the (pi; 0)/(0; pi) point, strongly suggests that the reason for this renormalization in YBCO-123 is coupling of the electronic subsystem to spin resonance, similar to the case of Bi-2212.

When a high-temperature superconductor transitions in the presence of a magnetic field, the resistance does not immediately go to zero. Instead, the transition is smoothed by the resistive effects of moving vortices so that perfect conductivity isn't reached until several K below Tc, where the vortices have frozen into a glass state. The properties of this glass depend mainly on the amount and correlation between impurities in the superconductor. This report describes a study of the glass transition in an YBCO thin film rich in uncorrelated impurities. The aim was to use sensitive resistive measurements to determine which of two models of this transition, the vortex glass model and the vortex molasses model, is more accurate. After evaluation, both models are found lacking and a third model is put forth. This model, which resembles the normal distribution, excellently describes the YBCO thin film case. When the new model is tested against data from similar measurements, an YBCO single crystal and a TlBaCaCuO thin film, the results are mixed. Further study is suggested to determine the model's range of validity.

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Thesis (PhD (Electrical and Electronic Engineering))--University of Stellenbosch, 2010. / Dissertation presented for the degree of Doctor of Philosophy in Engineering at the University of Stellenbosch / ENGLISH ABSTRACT: Planar weak link structures, such as micro-bridges, variable thickness bridges and nanobridges, have always attracted a lot of attention. Their potential to behave as real Josephson elements make them useful devices, with numerous applications. Powerful techniques, such as focused ion-beam and electron-beam lithography, were successfully used and are well understood in planar weak link structure fabrication. In this dissertation the results of an experimental study on planar weak link structures are presented. For the first time these structures have been successfully fabricated using AFM nanolithography on hard high-temperature superconducting YBCO tracks, where diamond coated silicon tips were used as a ploughing tool. Superconducting YBCO thin films were deposited on different substrates, using inverted cylindrical magnetron sputtering. The films were used to fabricate micro-bridges, variable thickness bridges and nano-bridges, by using conventional photolithography, argon ion-beam milling and AFM nanolithography. The measured I-V characteristics of the fabricated micro-bridges (width down to 1.9 µm), variable thickness bridges (thickness down to 15 nm) and nano-bridge (width down to 490 nm) showed well defined DC and AC Josephson effect characteristics. For better understanding of the behaviour of these types of weak links, critical current versus temperature measurements, and magnetic field modulation of the critical current measurements, were also performed, with the results and discussions given inside the chapters. The major challenges that were experienced in the laboratory during the fabrication processes and the operation of the fabricated devices are also discussed, with the solutions given where appropriate. / AFRIKAANSE OPSOMMING: Swak-skakel vlakstrukture, soos mikrobr.ue, br.ue met veranderlike dikte en nanobr.ue, het nog altyd baie aandag getrek. Hul het die potensiaal om soos werklike Josephson-elemente te kan funksioneer en is, as gevolg hiervan, nuttige toestelle met veelvuldige toepassings. Kragtige tegnieke, soos gefokuste ioonstraal- en elektronstraal litografie, is suksesvol gebruik en word goed verstaan in die vervaardiging van swak-skakel vlakstrukture. In hierdie proefskrif word die resultate van ¡¦n eksperimentele studie van swak-skakel vlakstrukture voorgel.e. Vir die eerste keer is hierdie strukture suksesvol vervaardig, deur gebruik te maak AFMnanolitografie op harde, ho¡Le-temperatuur supergeleier YBCO (Yttrium Barium Koperoksied) spore, waar diamantbedekte silikonpunte gebruik is as ploeginstrument. ¡¦n Dun lagie van supergeleidende YBCO is op verskillende substrate gedeponeer, deur gebruik te maak van omgekeerde silindriese magnetron verstuiwing. Die dun lagies is gebruik in die vervaardiging van mikrobr.ue, br.ue met veranderlike dikte en nanobr.ue, deur die gebruik van gewone fotolitografie, argon-ioonstraal frees en AFM nanolitografie. Die gemete I-V eienskappe van die vervaardigde mikrobr.ue (met breedte so laag as 1.9 µm), veranderlike-dikte br.ue (dikte tot 15 nm) en nanobr.ue (breedte so min as 490 nm) toon goed gedefinieerde GS en WS eienskappe van die Josephson-effek. Ten einde die gedrag van hierdie tipes swak-skakels beter te kan verstaan, is metings gedoen van kritieke stroom teenoor temperatuur, asook magnetiese veld modulasie van die kritieke stroom. Hierdie resultate en besprekings daarvan word binne die toepaslike hoofstukke aangebied. Die grootste uitdagings wat in die laboratorium, sowel as met die toetsing van die vervaardigde toestelle ondervind is, word ook bespreek. Waar moontlik, word toepaslike oplossings voorgestel.

High temperature superconductors (HTS) and solid oxide fuel cells (SOFCs) both offer the possibility for dramatic improvements in efficiency in power applications such as generation, transmission and use of electrical energy. However, production costs and energy losses prohibit widespread adoption of these technologies. This thesis investigates low-cost methods to tailor the structures of HTS wires and SOFCs to reduce these energy losses. Section I focusses on methods to produce filamentary HTS coated conductors that show reduced AC losses. This includes spark-discharge striation to pattern existing HTS tapes and inkjet printing of different filamentary architectures. The printed structures are directly deposited filaments and ‘inverse’ printed tracks where an initially deposited barrier material separates superconducting regions. Furthermore, the concept and first stages of a more complex ‘Rutherford’ cable architecture are presented. Additionally, Section I investigates how waste material produced during the manufacture of an alternative low-AC loss cable design, the Roebel cable, can be used to make trapped field magnets that produce a uniform magnetic field profile over a large area. This trapped field magnet work is extended to study self-supporting soldered stacks of HTS tape that demonstrate unprecedented magnetic field uniformity. Section II looks at how nanostructuring porous SOFC electrodes via solution infiltration of precursors can improve long-term stability and low temperature performance. Inkjet printing is utilised as a scalable, low-cost technique to infiltrate lab-scale and commercial samples. Anode infiltration via inkjet printing is demonstrated and methods to increase nanoparticle loading beyond ~1 wt% are presented. Symmetric cells with infiltrated cathodes are shown to have improved performance and stability during high temperature aging. Additionally, the sequence of solution infiltration is found to be important for samples dual-infiltrated with two different nanoparticle precursors.

this work deals with crystallographic texture analysis of ceramics and thin layers of YBCO, and with the texture effect on the superconducting properties, particularly on the critical current density.<br />X-ray diffraction was chiefly used, completed for specific samples by neutron diffraction and electron back scattering diffraction. We have developed the defocusing corrections needed for quantitative texture analysis of thin samples and multilayers, in the case of x-ray diffraction using the Schulz reflection geometry. Experimental conditions adapted to highly textured samples are detailed.<br />Highly textured bulk samples were elaborated by derived melt texture growth techniques. Under certain conditions, the texture is developed from the free surfaces of the sample. Generally, bulk textured samples exhibit two nearly oriented textures, which explain the relatively low measured Jcs. The "green" Y2BaCuO5 phase does not seemed textured in these samples. The texture and Jc relationship is clearly demonstrated in bulk samples, even if a quantitative description of the texture influence is still hard to establish.<br />In thin hetero-epitaxial layers, the observation of typical textures described in terms of near coincidence site lattices, allowed a quantitative texture analysis. Using a MgO(001) oriented single crystalline substrate, the interface region is very perturbated, giving rise to relatively bad superconducting properties of the layers (compared with other substrates offering better lattice mathching with YBCO). The Jc is quantitatively correlated to the texture degree of the films. Only weak volumic percentages of misorientations are necessary to strongly diminish Jc, while CuO precipitates can act in the opposite way.<br />Finally, texture analysis of experimental dispositives allows a quantitative characterisation of artificially generated misorientations, needed for the elaboration of Josephson junctions.

High-temperature superconductors have been intensely studiedsince the discovery, almost 20 years ago. Their layeredstructure, extreme type-II behavior, large anisotropy, andstrong fluctuations have led to a large number of new andinteresting problems. In this work the resistive transitionhave been studied in YBa2Cu3O7-δsingle crystals, from the superconductingfluctuations above Tcdown to the vortex matter near the onset of resistivity.Superconducting fluctuations above Tcare suitably studied by measuring themagnetoconductivity Δσ = 1/ρ(B)-1/ρ(0).Such experiments were performed on untwinned, optimally dopedsamples, forI||ab. ForH||c, fluctuations in the CuO-chains was not important, andfluctuations in the CuO2 planes seemed isotropic. The in-planecoherence length anisotropy was determined to be close tounity. ForH||ab, Δσbdepended on the field direction, indicating an effect ofthe chains on the magnetoconductivity in thisconfiguration. The nature of the vortex phase below Tccan be examined through vortex-correlation measurements.Using a modified pseudo-flux transformer (PFT) geometry forin-plane experiments, the magnetic field could be appliedparallel to both the a, b, and c axis. A strong Lorentz forcewas detected, and observed to be a requirement for the meltingstep feature. The resistive anisotropy close to Tcwas strongly field and temperature dependent, which wasexplained by an almost isotropic fluctuation contribution. The effect of anisotropy on vortex-liquid correlation infieldsH||ab, was studied byc-axis transport measurements in oxygen-deficientsamples, using a PFT configuration. Increasing disordersuppressed both longitudinal and transverse correlation. Inheavily underdoped samples, vortex correlation was enhanced formagnetic fields exactly aligned with theabplane because of the strong anisotropy. Also, thesolid-to-liquid transition temperature became nearly fieldindependent. Attempts were also made to connect the fluctuation regime ofthe resistive transition with the vortex region,by accountingfor critical fluctuations close to the transition. Keywords:high-temperature superconductors, YBa2Cu3O7-δ, untwinned YBCO, oxygen deficiency,superconducting fluctuations, magnetoconductivity, in-planeanisotropy, vortex liquid, vortex solid, vortex correlation

With the recent development of in situ transmission electron microscopy (TEM) characterization techniques, the real time study of property-structure correlations in nanomaterials becomes possible. This dissertation reports the direct observations of deformation behavior of Al2O3-ZrO2-MgAl2O4 (AZM) bulk ceramic nanocomposites, strengthening mechanism of twins in YBa2Cu3O7-x (YBCO) thin film, work hardening event in nanocrystalline nickel and deformation of 2wt% Al doped ZnO (AZO) thin film with nanorod structures using the in situ TEM nanoindentation tool. The combined in situ movies with quantitative loading-unloading curves reveal the deformation mechanism of the above nanomaterial systems. At room temperature, in situ dynamic deformation studies show that the AZM nanocomposites undergo the deformation mainly through the grain-boundary sliding and rotation of small grains, i.e., ZrO2 grains, and some of the large grains, i.e., MgAl2O4 grains. We observed both plastic and elastic deformations in different sample regions in these multi-phase ceramic nanocomposites at room temperature. Both ex situ (conventional) and in situ nanoindentation were conducted to reveal the deformation of YBCO films from the directions perpendicular and parallel to the twin interfaces. Hardness measured perpendicular to twin interfaces is ~50% and 40% higher than that measured parallel to twin interfaces, by ex situ and in situ, respectively. By using an in situ nanoindentation tool inside TEM, dynamic work hardening event in nanocrystalline nickel was directly observed. During stain hardening stage, abundant Lomer-Cottrell (L-C) locks formed both within nanograins and against twin boundaries. Two major mechanisms were identified during interactions between L-C locks and twin boundaries. Quantitative nanoindentation experiments recorded during in situ experiments show an increase of yield strength from 1.64 to 2.29 GPa during multiple loading-unloading cycles. In situ TEM nanoindentation has been conducted to explore the size dependent deformation behavior of two different types (type I: ~ 0.51 of width/length ratio and type II: ~ 088 ratio) of AZO nanorods. During the indentation on type I nanord structure, annihilation of defects has been observed which is caused by limitation of the defect activities by relatively small size of the width. On the other hand, type II nanorod shows dislocation activities which enhanced the grain rotation under the external force applied on more isotropic direction through type II nanorod.