The discovery of high-temperature superconductivity in 1986 in copper-oxide materials have opened up new avenues to investigate new families of quantum materials that were previously not known. Understanding the mechanism of superconductivity in high-T[sub]c superconductors has been an important research theme in condensed matter physics, as it is believed to be essential to realize the next generation engineered materials that become superconducting at room temperature. Discovered in 2006, iron based superconductors are a new addition to the family of high-T[sub]c superconductors, these materials exhibit several interesting properties and show some vivid similarities with cuprates and other families of high-temperature superconductors. In this thesis, I will present the spin-polarized scanning tunneling microscopy (SPSTM) study carried out on the parent compound of iron chalcogenide high temperature superconductor Fe[sub](1+y)Te to investigate the bi-collinear antiferromagnetic order. Magnetic tips in this work are prepared using a novel preparation technique by picking up excess iron atoms and clusters of FeTe from the surface of the sample. Next, I will present the SP-STM results obtained in the spin glass phase of Fe[sub](1+y)SeₓTe₁₋ₓ visualizing the interplay between the short ranged bi-directional bi-collinear antiferromagnetic order and superconductivity at the atomic scale. In this thesis, I will also present the scanning tunneling microscopy and spectroscopy (STM/STS) study of the native and engineered defect bound states in the iron-pnictide superconductor LiFeAs. This study addresses the pairing symmetry of the superconducting order parameter and understanding of dip-hump features seen in STM spectra outside the superconducting gap in iron pnictide superconductor LiFeAs.

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The capability to probe the dispersion of elementary spin, charge, orbital, and lattice excitations has positioned resonant inelastic x-ray scattering (RIXS) at the forefront of photon science. In this work, we will investigate how RIXS can contribute to a deeper understanding of the orbital properties and of the pairing mechanism in unconventional high-temperature superconductors. In particular, we will show how direct RIXS spectra of magnetic excitations can reveal long-range orbital correlations in transition metal compounds, by discriminating different kind of orbital order in magnetic and antiferromagnetic systems. Moreover, we will show how RIXS spectra of quasiparticle excitations in superconductors can measure the superconducting gap magnitude, and reveal the presence of nodal points and phase differences of the superconducting order parameter on the Fermi surface. This can reveal the properties of the underlying pairing mechanism in unconventional superconductors, in particular cuprates and iron pnictides, discriminating between different superconducting order parameter symmetries, such as s, d (singlet pairing) and p wave (triplet pairing).

Fe chalcogenides : The discovery of superconductivity in Fe based compounds trig- gered an intense research activity in this field with significant importance given to material synthesis. As a result, numerous materials falling into four major classes and sharing similarities in physical properties were synthesized and investigated. In spite of subtle differences, all of them share many common features like crystal symmetry, magnetic ground state, close resemblance in phase diagram etc. Fe super- conductors are broadly classified into Fe pnictides (with Fe − pnictogen layer) and Fe chalcogenides (with Fe − chalcogen layer) in which the binary Fe chalcogenides possess the simplest crystal structure. The distinct magnetic and superconducting properties make them interesting candidates for research. Detailed study on such systems demand high quality single crystals. This thesis discusses single crystal growth and properties of Fe1+yTe1−xSex. Struc- tural, magnetic, superconducting and thermal properties of pristine and substituted compounds are explored. A characteristic feature associated with binary chalco- genides is the presence of excess Fe in the interstitial sites represented by y in the chemical formula. By fine tuning the composition, the effect of interstitial Fe on various physical properties can be analyzed. The current work deals with the influence of interstitial excess Fe on the structural, magnetic and superconducting properties of the parent compound Fe1+yTe and Se substituted Fe1+yTe1−xSex. The results are organized into eight chapters; an outline of each chapter is given below. Chapter 1 gives an introduction to the broad field of Fe superconductors. A de- tailed literature review providing comparison of Fe pnictides with chalcogenides is included. The background of the current work is discussed with reference to the im- portant aspects of crystal structure and its relation to the ordered ground states. An overview of the important theories on magnetic ordering and superconducting pair- ing is provided. In the later part, a generic phase diagram along with the individual phase diagrams of important systems are discussed. This is followed by a discus- sion of the characteristic properties of iron chalcogenides and different methods of bulk synthesis. The chapter is concluded with a note on the motivation behind the present work. Chapter 2 discusses the crystal growth techniques and experimental methods used in the present work. The basic working principles are briefly explained. Chapter 3 provides a detailed discussion of the single crystal growth procedure, its customization and basic characterization. Single crystals of all compositions un- der discussion are grown by a modified horizontal Bridgman method. Material preparation, growth parameters and overall temperature profile of crystal growth process are described. Single crystalline nature of the as-grown crystals is con- firmed with Laue scattering technique. All crystals show tetragonal symmetry at room temperature. The approximate crystal orientation is deduced by indexing the X-ray diffraction pattern of the cleaved crystals. The diffraction patterns exhibit a set of (00l) peaks. A detailed composition analysis is performed on the samples. The sample properties are very sensitive to composition and careful estimation is per- formed by conducting repeated measurements at multiple points on the samples under study. Chapter 4 deals with superconducting and magnetic properties of Fe1+yTe0.5Se0.5. Single crystals of two different Fe concentration, y=0.04 and 0.09 are grown in which the concentration of Se and Te are maintained close to 0.5. Among binary Fe chalcogenides, half substituted iron telluride shows the highest TC (15 K) at ambient pressure. Accordingly, this composition is chosen to evaluate the role of Fe concentration in modulating the superconducting behavior. Two different batches of both the samples are grown, one set containing small amounts of impurity phases and the other, representing a pure primary phase. Resistivity measurements performed on both compositions, y=0.04 and 0.09, show onset of superconductivity near 15 K. In the normal state above TC, the temperature derivative of resistivity dρ/dT changes from positive to negative as the excess Fe concentration rises. At higher Fe concentrations, a log 1/T divergence is discernible in the normal state. The contribution of interstitial Fe to superconductivity has been analyzed using magnetization measurement techniques. An increase in the width of superconducting transition is seen in all measurements as the Fe content increases. The superconducting volume fraction estimated from susceptibility data demonstrates that high concentration of Fe is not favorable to superconductivity. The upper and lower critical field are esti- mated from electric resistivity data (in applied magnetic field) and magnetization isotherms respectively. Comparison of the lower critical field between two compo- sitions strengthens the argument that higher excess Fe leads to suppression of super- conductivity. The second set of crystals with impurity phases reveals an anomalous magnetization peak near 125 K. The results from resistivity, DC magnetization and ac susceptibility are compared. Chapter 5 addresses the influence of excess Fe on the ordered ground state. The antiferromagnetic parent compound, Fe1+yTe single crystals, are also grown using the same procedure. It is proposed that excess Fe occupying the interstitial sites possess local moments which could interact with the magnetic phases. In an at- tempt to understand their magnetic properties in detail, single crystals are grown with y=0.06, 0.09, 0.11, 0.12, 0.13 and 0.15. Fe1+yTe undergoes magnetostructural transition at TN=67 K. As the concentration of Fe varies from 0.06 to 0.13, a marked suppression of TN occurs from 67 K to 56 K. Moreover, a single first order transi- tion is seen to split into two at the critical concentration, y=0.12. The derivative plot of magnetization and specific heat data clearly illustrate two well-separated peaks. The two transitions are denoted as TN=57 K and TS=46 K. TN here is identified as a second order transition and TS as a first order transition. The second order transi- tion is evident from the λ-like nature of the peak in the specific heat measurement. The first order transition is associated with a large thermal hysteresis in the heat- ing and cooling cycle. Raw data from the heat capacity calorimeter gives a clear hint towards the first order nature of TS. As the composition of Fe rises further, the multiple transitions subside and disappear. For higher concentration, y=0.15, a sin- gle continuous phase transition is observed. Impurity free, pure phase is noticed in most of the samples as evident in powder X-ray diffraction and bulk magneti- zation measurements. The thermal data of various compositions are analyzed and compared. Electrical resistivity data clearly reveals the shift in phase transition and the presence of multiple transitions. Unlike Fe1+yTe1−xSex, all compositions here display similar behavior above TN, irrespective of the concentration of excess Fe. Chapter 6 devotes special emphasis to the evolution of structural and magnetic properties of the critical composition, Fe1.12Te where multiple transitions are ob- served. The low temperature structure of the crystal is studied in detail using syn- chrotron powder X-ray diffraction. The data in the vicinity of the two transitions, TN and TS are carefully analyzed. The room temperature crystal structure belongs to tetragonal symmetry with P4/nmm space group, where it is paramagnetic. As the sample is cooled to just below TN, a magnetostructural transition occurs from tetragonal to orthorhombic space group Pmmn. Below TN, the XRD pattern of the tetragonal (200) peak splits into (200) and (020) representing an orthorhombic distortion. The second transition is observed at TS where the orthorhombic struc- ture undergoes a monoclinic distortion, to P21/m. Below TS, a mixed phase of or thorhombic and monoclinic structures are present. The powder diffraction studies are supplemented with thermodynamic measurements. From specific heat analy- sis, the different contributions and the change in entropy across the transitions are estimated. Linear thermal expansion study has confirmed the two structural transi- tions. The changes occurring in lattice parameters, bond distances, bond angles and unit cell volume as a function of temperature are calculated using powder pattern refinement. Synchrotron data, linear thermal expansion and thermodynamic mea- surement results all point to strong magnetostructural coupling in this material. A temperature-composition phase diagram is formulated using results obtained from different Fe compositions. Transition temperature is plotted as a function of excess Fe content, highlighting its role in determining the structural and magnetic phases in Fe1+yTe. Chapter 7 deals with the magnetic and superconducting properties of Se substi- tuted Fe1+yTe1−xSex. Single crystals are grown by carefully varying the concen- tration of Se from x=0.02 to 0.25 while keeping the nominal composition of excess Fe more or less same. In this work, focus is given to Fe-rich, selenium substituted compositions. The intention is to explore how Se substitution affects the multiple transitions observed in Fe1.12Te. At 2% Se substitution, the split peaks are evident with a slight shift in temperature. The temperature interval between the two tran- sitions decreases in comparison to the pristine compound. For further increases in Se concentration, instead of two well separated peaks, a weak broad hump is ob- served. For compositions with x >0.10, long range magnetic ordering is suppressed. As x increases above 0.15 the electrical resistivity drops indicating the onset of su- perconductivity. However, in the composition range 0.15 ≤ x ≤ 0.25, neither long range magnetic order nor bulk superconductivity is present. Alternately, weak magnetic transitions above the superconducting transition are visible. The transport and magnetic properties are similar to that observed in Fe1.09Te0.5Se0.5. By tuning the Se composition in Fe-rich samples, the magnetic and structural transitions, originally seen in the parent compound are suppressed. The emergence of superconductivity is also discussed. The last section of the chapter provides the modified phase diagram as a function of Se concentration, combining all compositions discussed in the thesis. This gives a detailed description of Fe chalcogenides in the composition range, x=0 to 0.5 with special emphasis on Fe rich samples. The different regions in the phases diagram describe the peculiar properties of Fe chalcogenides. Chapter 8 concludes the thesis with general conclusions pertaining to various observations made in the different chapters. Prospects for future work are briefly outlined.

The capability to probe the dispersion of elementary spin, charge, orbital, and lattice excitations has positioned resonant inelastic x-ray scattering (RIXS) at the forefront of photon science. In this work, we will investigate how RIXS can contribute to a deeper understanding of the orbital properties and of the pairing mechanism in unconventional high-temperature superconductors. In particular, we will show how direct RIXS spectra of magnetic excitations can reveal long-range orbital correlations in transition metal compounds, by discriminating different kind of orbital order in magnetic and antiferromagnetic systems. Moreover, we will show how RIXS spectra of quasiparticle excitations in superconductors can measure the superconducting gap magnitude, and reveal the presence of nodal points and phase differences of the superconducting order parameter on the Fermi surface. This can reveal the properties of the underlying pairing mechanism in unconventional superconductors, in particular cuprates and iron pnictides, discriminating between different superconducting order parameter symmetries, such as s, d (singlet pairing) and p wave (triplet pairing).

We study unconventional superconductivity in two-dimensional systems. Unbiased numerical calculations within two-dimensional Hubbard models have found no evidence for long-range superconducting order. Most of the two-dimensional theories suggest that the superconducting state can be obtained by destabilizing an antiferromagnetic or spin-liquid insulating state. An antiferromagnet is a half-filled system because each site has one electron or hole. However, in anisotropic triangular lattices, numerical calculation finds pairing enhancement at quarter-filling but no long-range superconducting order. Many organic superconductors are dimerized in nature. Such a dimer lattice is effectively half-filled because each dimer has one electron or hole. Some theories suggest that magnetic fluctuation in such a system can give superconductivity. However, at zero temperature, we performed density matrix renormalization group (DMRG) calculations in such a system, and we find no superconducting long-range order. We also find that the antiferromagnetic order is not necessary to get a superconducting state. Failure in explaining superconductivity in two-dimensional systems suggests that only repulsive interactions between electrons are not sufficient, and other interactions are required. The most likely candidate is the electron-phonon interaction. However, existing theories of superconductivity emphasize either electron-electron or electron-phonon interactions, each of which tends to cancel the effect of the other. We present direct evidence from quantum Monte Carlo calculations of cooperative, as opposed to competing, effects of electron-electron and electron-phonon interactions within the frustrated Hubbard Hamiltonian, uniquely at the band-filling of one-quarter. Bond-coupled phonons and the onsite Hubbard U cooperatively reinforce d-wave superconducting pair-pair correlations at this filling while competing with one another at all other densities. Our work further gives new insight into how intertwined charge-order and superconductivity appear in real materials.

Condensed matter physics is a rapidly evolving field of research enriched with the synthesis of new materials exhibiting a bewildering variety of phenomena and advances in experimental techniques. Over the years, discoveries and innovations in electronic systems have emphasized the crucial role played by correlations among electrons behind many of the observed unusual properties and have posed serious challenges to the physics community by exposing the lack of well-controlled theoretical methods to study the class of materials known as strongly correlated electronic systems. In these systems, known theoretical techniques typically fail to capture the essential features of the many-body ground state and finite temperature properties of the systems as typical electronic interaction energies are of order of or larger than the kinetic energies. The study of strongly correlated electronic systems went through a revolution in the 1980s and 1990s after the discovery of superconductivity inorganic compounds, in heavy fermion systems and ultimately in copper oxides, referred to as cuprates, by Bednorz and Muller. In particular, the pursuit of understanding the mysterious origin of superconductivity in the cuprates and other associated strange phenomena has fascinated the condensed matter community over last two and half decades leading to most of the important unsolved, and probably interconnected, problems of quantum condensed matter physics such as the metal-insulator transition in low dimensions breakdown of Fermi liquid theory, the origin and behavior of unconventional superconductivity, quantum critical points, electronic in homogeneities and localization in interacting systems. This thesis is devoted to the study of some of the aspects of high-temperature superconductivity and associated phenomena in cuprates. In what follows, I give an overview of the organization of the thesis in to different chapters and their contents. For setting up the stage, in Chapter 1, I give a brief account of some of the remarkable phenomena and properties observed in strongly correlated electronic matter and their salient features, that continue to draw much attention and excitement in current times. The peculiarity of the state of affairs in these systems is emphasized and motivated in the background of the paradigmatic Landau Fermi liquid theory and Hubbard model, the minimal model that is expected to capture the quintessence of electronic strong correlation. In Chapter 2, starting with a brief historical account of the discovery of superconductivity in cuprates, the crystal structure of these materials, their chemical realities and basic electronic details are reviewed. This is followed by a survey of the phase diagram of cuprates, doped with, say, x number of holes per copper site, and a plethora of experimental findings that constitute the high-c puzzle. Characteristics of various observed phases, such as the superconducting, pseudo gap and strange metal phases, are discussed on the basis off acts accumulated through various experimental probes, e.g. nuclear magnetic resonance(NMR), neutron scattering, specific heat, transport and optical conductivity measurements as well as photo emission, tunnelling and Raman spectroscopies. As elucidated, these experiments point toward the need for an unconventional mechanism of superconductivity in cuprates and, more so, for the description of the rather abnormal high-temperature normal state that is realized above the superconducting transition temperature c. Keeping in mind the fact that there is no consensus even about the minimal microscopic electronic model, I review two models, namely the three band model and the t - J model; various approximate treatments of these models have dominated the theoretical developments in this field. A large number of theoretical pictures have been proposed based on different microscopic, semi-microscopic and phenomenological approaches in the past two decades for explaining the genesis of the observed strange phenomena in high-c cuprates. I include brief discussions on only a few of them while citing relevant references. As mentioned above, a variety of approximate microscopic theories, based on both strong and weak coupling approaches, as well as numerical techniques have been tried to understand the cuprate phase diagram and capture the aspects of strong correlations in-built in Hubbard and t -J models. On the other hand, in conventional superconductors and, in general, for the study of phase transitions, phenomenological Ginzburg-Landau(GL) functionals written down from very general symmetry grounds have provided useful description for a variety of systems. Specially, Ginzburg-Landau theory has been proven to be complementary to the BCS theory for attacking a plethora of situations in superconductors, e.g., in homogeneities, structures of an isolated vortex and the vortex lattice etc. The GL functional has found wide applicability for the study of vortex matter in high-c superconductors as well. Inspired by the success of this type of phenomenological route, we propose and develop in Chapter 3 an approach, analogous in spirit to that of Ginzburg and Landau, for the superconducting and pseudogap phases of cuprates. We encompass a large number of well known phenomenologies of cuprate superconductivity in the form of a low-energy effective lattice functional of complex spin-singlet pair amplitudes with magnitude Δm and phase m, i.e. m =Δm exp(i m), that resides on the Cu-Cubonds(indexed by m)of the CuO2 planes of cuprates. The functional respects general symmetry requirements, e.g. the -wave symmetry of the superconducting order parameter as found in experiments. The assumptions and the specific physical picture behind such an approach as well as the key empirical inputs that go into it are discussed in this chapter. We calculate the superconducting transition temperature c and the average magnitude of the local pair amplitude, Δ= (Δm), using single-site mean-field theory for the model. We show that this approximation leads to general features of the doping-temperature(x - T )phase diagram in agreement with experiment. In particular, we find a phase coherent superconducting state with d-wave symmetry below a parabolic Tc (x) dome and a phase incoherent state with a perceptible local gap that persists up to a temperature around which can be thought of as a measure of the pseudogap temperature scale T* . Further, effects of thermal fluctuations beyond the mean-field level are captured via Monte Carlo(MC) simulations of the model for a finite two-dimensional (2D) lattice. We exhibit results for Tc obtained from MC simulations as well as that estimated in a cluster mean field approximation. Based on our picture we remark on contrasting scenarios proposed for the doping dependence of the pseudogap temperature. Chapter 4 describes fluctuation phenomena related to pairing degrees of freedom and manifestations of these effects in various quantities of interest, e.g. superfluid density, specific heat etc., at finite temperature. Fluctuation effects have been studied in detail in superconductors over the years and pursued mainly using either the conventional GL functional or the BCS-framework at a microscopic level. However, the picture, in which the pseudogap phase is viewed as one consisting of bond-pairs with a d-wave symmetry correlation length growing as T approaches Tc, implies fluctuation phenomena of quite a different kind, as we discuss here. The contribution of the bond-pair degrees of freedom to thermal properties is obtained here from the lattice free-energy functional using MC simulation, as mentioned in the preceding paragraph. The results for the superfluid density or superfluid stiffness ps, a quantity measured e.g. via the penetration depth, are discussed. As shown, its doping and temperature dependence compare well with experimental results. In this chapter, I also report the calculation of the fluctuation specific heat Cv(T) and find that there are two peaks in its temperature dependence, a sharp one connected with Tc (ordering of the phase of m)and a relatively broad one(hump)connected to T* (rapid growth of the magnitude of Δm). The former is specially sensitive to the presence of a magnetic field, as we find in agreement with experiment. Vortices are relevant excitations in a superconductor and, in particular, in 2D orquasi-2D systems vortices influence the finite temperature properties in a major way. The results for the temperature dependence of vortex density obtained in the MC simulation of the GL-like model are also mentioned in Chapter 4. I report an estimate of the correlation length as well. These results might have relevance for the large Nernst signal observed over a broad temperature range above c in cuprates, as pointed out there. Properties of an isolated vortex and collective effects arising due to interaction between vortices are of much significance for understanding mixed state of type-II superconductors and thus of cuprates. The superconducting order is destroyed in the core region around the centre of a vortex and the vortex core carries signatures of the normal state in a temperature regime where it is generally unattainable due to occurrence of superconductivity. As mentioned in Chapter 5, vortex properties(e.g. electronic excitation spectrum at the vortex core) in BCS superconductors have been explored theoretically, at a microscopic level through the Bogoliubov-deGennes(BdG) theory as well as using the Ginzburg-Landau functional. However, properties of vortices in cuprate superconductors have been found to be much more unusual than could possibly be captured by straightforward extensions of BCS theory to a -wave symmetry case. Chapter 5 briefly reviews the experimental findings on vortices in the superconducting state of cuprates, mainly as probed by Scanning Tunnelling Microscopy(STM) as well as from other probes such as NMR, neutron scattering, SR etc. I discuss some of the consequences of our GL-like functional regarding vortex properties, namely that of the vortex core and the region around it. We use our model to find Δm and m at different sites m for a 2π vortex whose core is at the midpoint of a square plaquette of Cu lattice sites. The vortex is found to change character from being primarily a phase or Josephson vortex for small x to a more BCS-like or Abrikosov vortex with a large diminution in the magnitude Δm as one approaches the vortex core, for large . Here I do not make any direct comparison with experimental data but discuss implications of our results in the background of existing experimental facts. Unravelling the mysteries of high-Tc cuprates should necessarily involve the understanding of electronic excitations over a broad regime of doping and temperature encompassing the pseudo gap, superconducting and strange metal states. A phenomenological theory which aims to describe the pseudo gap phase as one consisting of preformed bond-pairs, is required to include both unpaired electrons and Cooper pairs of the same electrons coexisting and necessarily coupled with each other. In our Ginzburg Landau approach only the latter are explicit, while the former are integrated out. However, effects connected with the pair degrees of freedom are often investigated via their coupling to electrons, one very prominent examples being Angle Resolved Photoemmision Spectroscopy(ARPES),in which the momentum and energy spectrum of electrons ejected from the metal impinged by photons is investigated. In Chapter 6, we develop a unified theory of electronic excitations in the superconducting and pseudo gap phases using a model of electrons quantum mechanically coupled to spatially and temporally fluctuating Cooper pairs(the nearest neighbour singlet bond pairs). We discuss the theory and a number of its predictions which seem to be in good agreement with high resolution ARPES measurements, which have uncovered a number of unusual spectral properties of electrons near the Fermi energy with definite in-plane momenta. We show here that the spectral function of electrons with momentum ranging over the putative Fermi surface(recovered at high temperatures above the pseudogap temperature scale) is strongly affected by their coupling to Cooper pairs. On approaching Tc i.e. the temperature at which the Cooper pair phase stiffness becomes nonzero, the inevitable coupling of electrons with long-wavelength(d-wave symmetry) phase fluctuations leads to the observed characteristic low-energy behavior as reported in Chapter 6. Collective d-wave symmetry superconducting correlations develop among the pairs with a characteristic correlation length ξ which diverges on approaching the continuous transition temperature Tc from above. These correlations have a generic form for distances much larger than the lattice spacing. As we show here, the effect of these correlations on the electrons leads, for example, to a pseudogap in electronic density of states for T > T c persisting till T* , temperature-dependent Fermi arcs i.e. regions on the Fermi surface where the quasiparticle spectral density is non zero for a zero energy excitation and to the filling of the antinodal pseudogap in the manner observed. Further, the observed long-range order(LRO) below c leads to a sharp antinodal spectral feature related to the non zero superfluid density, and thermal pair fluctuations cause a deviation(‘bending’) of the inferred ‘gap’ as a function of k from the expected d-wave form (cos kxa - cos kya). The bending, being of thermal origin, decreases with decreasing temperature, in agreement with recent ARPES measurements. I conclude in Chapter 7 by mentioning some natural directions in which the functional and the approach used here could be taken forward. The phenomenological theory proposed and developed in this thesis reconciles and ties together a range of cuprate superconductivity phenomena qualitatively and confronts them quantitatively with experiment. The results, and their agreement with a large body of experimental findings, strongly support the mechanism based on nearest neighbor Cooper pairs, and emergence of long-range -wave symmetry order as a collective effect arising from short range interaction between these pairs. This probably points to the way in which high-c superconductivity will be understood.

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.

Orientador: Oscar Ferreira de Lima / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataglin / Made available in DSpace on 2018-09-24T18:20:24Z (GMT). No. of bitstreams: 1 Oliveira_TarcisoMesquitade_M.pdf: 3236428 bytes, checksum: f8799c6f4d4ac1a196ca499b0ad67efb (MD5) Previous issue date: 2005 / Resumo: Nesta tese estudamos possíveis origens do Segundo Pico da Magnetização (SPM) em supercondutores de alta e baixa temperatura crítica, usando amostras monocristalinas de Bi2Sr2CaCu2O8+d e de Nb. Para estudar o SPM fizemos medidas de curvas de magnetização M x H e suscetibilidade AC (vs. h e T). Através da aplicação de uma lei de escala, para suscetibilidade AC, obtivemos o expoente de creep na região do SMP. Em amostras de Nb verificamos os efeitos sobre as curvas M x H de não homogeneidades na amostra: deslocamentos de planos e oxigênio intersticial. Em amostras de alta pureza (bulk e monocristalina) observamos que as curvas M x H não apresentam o SPM nem o Efeito Pico, próximo a Hc2.Ao adicionar não homogeneidades nas amostras, através de dopagem com oxigênio e deformação por elongação, observamos o aparecimento do Efeito Pico, de instabilidades termomagnéticas e de anisotropia do campo Hc2. Em amostras de Bi2Sr2CaCu2O8+d observamos que o SPM está associado com a componente do campo aplicado paralela à direção c da rede cristalina, ou seja, que o SPM está relacionado com vórtices de Abrikosov e que vórtices Josephson parecem não influenciar no SPM. Ao clivarmos a amostra observamos que a intensidade do SPM se reduz e diminui a janela de temperatura onde ele é visto. Obtivemos o expoente de creep e observamos que na região do SPM o arrasto dos vórtices diminui em campos que antecedem o HSPM , mas na região de HSPM o arrasto aumenta rapidamente e depois volta a cessar. Interpretamos a variação brusca no expoente de creep como uma mudança de fase no sistema de vórtices, que passa de um estado de quase-rede para um estado emaranhado de vórtices / Abstract: In this thesis we have studied possible origins for the Second Magnetization Peak (SMP) in high and low critical temperature superconductors, using samples of Bi2Sr2CaCu2O8+d and Nb single crystals. To study the SMP we did measurements of magnetization curves (M x H) and AC susceptibility (vs. h and T). We obtained the creep exponent in the SMP region, using a scaling law for the AC susceptibility data. In the Nb samples we have verified the effects on M x H curves due to inhomogeneities like dislocations and interstitial oxygen. In high pure samples (bulk and single crystals) we have observed that the M x H curves do not present either SMP or Peak Effect, near H2c . By adding inhomogeneities in the samples, like doping them with oxygen or deforming by elongation, we have observed the appearance of Peak Effect, thermomagnetic instabilities, and H2c anisotropy. In Bi2Sr2CaCu2O8+d samples we have observed that the SMP is associated with the magnetic field component parallel to the crystal c direction of crystal net. This means that the SMP is related to Abrikosov¿s vortices and that Josephson¿s vortices seem not to influence the SMP. After cleaving the sample we observed that the SMP intensity was reduced and the temperature window were it occurs was decreased. We have obtained the creep exponent and have observed that in the SMP region the vortices creep diminish for fields below HSPM , but around HSPM the creep increases abruptly and after the SPM it decreases again. We have interpreted the abrupt creep exponent variation as a phase transition in the vortex system, that pass from a quasi-lattice state to an entangled vortex state / Mestrado / Física / Mestre em Física