The origin of the nematic phase in the iron-based superconductors is still unknown, and an understanding of its microscopic mechanism could have important implications on the unconventional superconductivity in these materials. This thesis describes a series of experiments using scanning tunneling microscopy (STM) and spectroscopy (STS) to visualize the nematic electronic structure in NaFe1-xCoxAs as a function of energy, temperature, strain, and doping. We first start with background material on the iron-based superconductors and the iron pnictides in particular. We then extensively explore the physical details of NaFe1-xCoxAs which is the main material of study in this thesis. Additional attention is paid to the electronic structure due to its relation to quasiparticle interference (QPI) measurements made with STS. The theoretical underpinnings of STM and STS are then derived as well as further details of QPI. Many of the experiments described in this thesis were performed on a custom-built, low temperature STM which the author helped build. We describe the design of this system and report on benchmarking tests that were used to characterize the system's performance. Both pristine, undoped NaFeAs and LiFeAs were measured by STM, and we compare and contrast these two materials which come from the same structural family. The electronic local density of states (LDOS) of NaFeAs was measured at various temperatures in all three phases of the material (tetragonal paramagnetic, orthorhombic paramagnetic, and orthorhombic spin density wave (SDW)). The electronic structure in the SDW phase is highly anisotropic. QPI signals in this phase are found to be well-explained by comparison to a joint density of states (JDOS) model using the reconstructed bandstructure fit to angle-resolved photoemission spectroscopy data. The electronic anisotropy is found to persist into the nominally tetragonal phase. This persistence arises from built-in crystallographic strain coupling to high amplitude, unidirectional, antiferroic fluctuations. These fluctuations renormalize the bare Green's function which gives rise to anisotropic scattering. We then describe the construction of a novel device created for variable-strain STS. Antiphase domains in NaFeAs are visualized and found to change in size as a function of unidirectional strain. These domains are tracked as a function of temperature and found to disappear at exactly the nematic transition temperature proving that this is the temperature at which long-range order is lost. By measuring Co-doped samples, we find that the domains disappear before optimal doping in an underdoped sample with superconducting transition temperature of 18 K. However, the electronic structure remains anisotropic implying that nematic fluctuations persist. These fluctuations are found even in overdoped samples and disappear with superconductivity at heavy doping.

Made available in DSpace on 2015-04-22T22:07:23Z (GMT). No. of bitstreams: 1 Dissertacao Final Marcio Gomes.pdf: 1885147 bytes, checksum: b68c412fc26d42c2c611a08b965c9c52 (MD5) Previous issue date: 2009-06-26 / Fundação de Amparo à Pesquisa do Estado do Amazonas / In this work, we study the properties of the high-temperature superconductors to analyze its rich phase diagram. We use the tight binding model within a mean-field formulation. The choice of the above model is due to its relative simplicity and also to verify the extent upto which it can provide satisfactory qualitative results. We use the tight binding model including terms upto second nearest hopping to study. These new superconductors in the underdoped and overdoped phases. We consider the charge density wave with d-wave symmetry (DDW), as a possible explanation for the pseudogap in the underdoped phase. The phase diagram of the cuprates are analyzed. We calculate some properties of these new materials, such as the chemical potential and specific heat and compare with other theories and experiments that attempt to explain superconductivity. We will also observe, to what extent the theory of mean field approximation is applicable. / Neste trabalho, estudaremos as propriedades que são universais aos supercondutes de altas temperaturas, analisando o seu rico diagrama de fase. Utilizamos-nos para isso, o modelo de tight binding numa formulação de campo m´edio. A escolha do modelo de tight binding foi devido ao interesse em verificar até que ponto, um método, relativamente simples que os demais, pode fornecer resultados quantitativos satisfatórios. Assumimos que o termo de hopping e as interações não locais ocorrem entre primeiros e segundos vizinhos. Consideraremos as ondas de densidade d (DDW), como possível explicação para o pseudogap no estado normal. Analisaremos o comportamento do diagrama de fase nos cupratos, o gap supercondutor e o pseudogap no estado normal e calcularemos algumas propriedades termodinâmicas, tais como, potencial químico e calor específico; fazendo um paralelo com outras teorias que tentam explicar a supercondutividade. Veremos, também, até que ponto a teoria de campo médio é útil.

Recentemente, os compostos não-centrossimétricos têm sido intensamente estudados devido à grande variedade de comportamentos magnéticos, os quais são induzidos pela ausência de centro de simetria na sua estrutura cristalográfica. Esta assimetria induz uma modificação na estrutura de bandas, causando um acoplamento spin-orbita antissimétrico quem é responsável por mecanismos de interação magnética inusuais. A supercondutividade em este tipo de compostos apresenta propriedades que divergem do comportamento esperado pela teoria BCS. Neste trabalho será apresentado um estudo sobre a investigação de supercondutividade em dois sistemas ternários, Y-Co-C e Lu-Ni-C particularmente nos compostos YCo1-xC2 e Lu1-xThNiC2. Estes compostos cristalizam na estrutura CeNiC2 de simetria ortorrômbica é do grupo espacial 38 Amm2 que não possui centro de inversão. As técnicas de caracterização usadas neste trabalho incluem medidas de magnetização, resistividade e calor especifico como funções da temperatura, assim como magnetização como função do campo magnético aplicado. O composto YCo0.7C2 é supercondutor com Tc = 4 K e exibe um comportamento que diverge da teoria BCS. As medidas realizadas neste trabalho sugerem que este material é um forte candidato como supercondutor não convencional, onde poderia existir uma mistura de contribuições nos canais singleto e tripleto. No caso do sistema Lu1-xThxNiC2 os resultados preliminares indicam claramente supercondutividade nos compostos dopados com Th, onde a composição Lu0.6Th0.4NiC2 apresenta uma Tc= 8 K. / In recent years, non-centrosymmetric compounds have attracted a great interest because of their wealth variety of topical behaviors, induced by the lack of the inversion center on the crystallographic structure. This asymmetry leads to a strong modification in the band structures, causing an antisymmetric spin-orbit coupling which is responsible for unusual magnetic interaction mechanisms. Superconductivity in compounds whose crystal structure lacks inversion symmetry are known to display intriguing properties that deviate from conventional BCS superconducting behavior. Here we report the analysis of the ternary systems Y-Co-C and Lu-Ni-C, We focused our analysis in the intermetallic compounds YCo1-xC2 and Lu1-xThxNiC2. Magnetization, resistivity, and heat capacity measurements on polycrystalline samples of noncentrosymmetric YCo0.7C2, showing clear evidence of bulk superconductivity with a critical temperature of Tc =4 K. Interestingly the specific heat of the superconducting state deviates from conventional exponential temperature dependence, which is suggestive of possible unconventional superconducting behavior in YCo0.7C2, similar to that seen in the isostructural and isoelectronic superconductor LaNiC2. Besides, these results strongly suggest that this material is a strong candidate of multiband superconductivity. In the case of the system Lu1- xThxNiC2 our results showed a clear superconducting signal in the Th doped samples, where the composition Lu0.6Th0.4NiC2 has a Tc= 8 K.

The development of an infrared instrument for the study of surface adsorption and high Tc superconductors is presented and its applicability is discussed. The use of and theoretical background for the metal light pipe multiple-reflection technique are discussed in comparison to other infrared devices. Measurements are carried out to determine its limitations with powdered substrates as well as its sensitivity limitation for thin films. The results of the two test measurements, adsorption of CO on MgO and investigation of the energy gap of high Tc superconductors, are presented. Comparisons of the spectra with the experimental and theoretical literature are presented where possible. The necessity for simultaneous infrared and vapor pressure isotherm measurements is discussed, and their instrumental realization is shown. Infrared measurements on the energy gap of high Tc superconductors are shown and compared with results form the literature.

Cette thèse a pour objectif de moduler la nanoporosité et la densité de gels Résorcinol-Formaldéhyde (RF) afin d’étudier leurs performances dans des applications industrielles telles que les supercondensateurs ou les matériaux d’isolation thermique. En effet, c’est leur texture, et plus particulièrement leur porosité, qui confère à ces matériaux de telles propriétés. Ce travail de thèse a consisté à étudier les paramètres de synthèse des systèmes RF en ajoutant un additif et à utiliser de nouveaux précurseurs. Notre choix s’est d’abord porté sur un polyélectrolyte cationique qui est susceptible de modifier les mécanismes de synthèse des gels ainsi que leur procédé de fabrication. Dans un second temps, l’emploi de nouveaux précurseurs a ouvert de nouvelles possibilités de synthèse de ces matériaux. Enfin, l’aspect sécuritaire du procédé a été abordé afin d’évaluer tous les risques liés à la synthèse de gels RF pour faciliter le transfert du procédé à plus grande échelle. / The objective of this thesis is to modulate the nanoporosity and the density of Resorcinol-Formaldehyde (RF) gels in order to study their performances in industrial applications such as supercapacitors or thermal insulation materials. Indeed, it is their texture and more particularly their porosity which give these materials such properties. This thesis work consisted in studying synthesis parameters of RF systems by adding an additive and using new precursors. We chose a cationic polyelectrolyte which is likely to modify the synthesis mechanisms of the gels as well as their elaboration process. In a second time, the use of new precursors offered new possibilities of synthesis of these materials. Finally, process security was discussed in order to evaluate all risks linked to RF gels synthesis in order to facilitate the scaling-up of the process.

No description available.

We carried out systematic neutron scattering experiments to investigate the magnetic properties and their relationship to the high-$T_c$ superconductivity, when the materials are tuned from their antiferromagnetic (AF) parent compounds to the superconducting regime. We observed resonance mode in the electron doped cuprate Nd$_{1.85}$Ce$_{0.15}$CuO$_4$, demonstrating that the resonance is a general phenomenon in cuprate superconductors regardless of hole- or electron-doping. In Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$, the local susceptibility displays two distinct energy scales that are broadly consistent with the bosonic modes revealed by scanning tunneling microscopy experiments. These results indicate the presence of very strong electron spin excitations couplings in electron doped cuprates. Shortly after the discovery of high-$T_c$ superconductivity in the Fe pnictides, we discovered that the magnetic phase diagram of CeFeAsO$_{1-x}$F$_x$ is remarkably similar to that of the cuprates. Besides CeFeAsO, similar magnetic and lattice structures are also observed in PrFeAsO and SrFe$_2$As$_2$ systems. Neutron scattering measurements show that in SrFe$_2$As$_2$, the spectrum of magnetic excitations consists of a Bragg peak at the elastic position, a spin gap, and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimated the effective magnetic exchange coupling using a Heisenberg model. In order to study the nature of the exchange interactions in the parent compound of Fe pnictides, we studied the high energy spin-wave excitations in CaFe$_2$As$_2$. Although the spin waves in the entire Brillouin zone can be described by an effective three-dimensional anisotropic Heisenberg Hamiltonian, the magnetism in this system is neither purely local nor purely itinerant; rather it is a complicated mix of the two. When the Fe pnictide is tuned into superconducting regime with doping, the low energy spin fluctuation is dominated by a resonance mode. In the optimally electron doped BaFe$_{1.9}$Ni$_{0.1}$As$_2$, application of a magnetic field that suppresses the superconductivity and superconducting gap energy also reduces the intensity and energy of the resonance. These results suggest that the energy of the resonance is proportional to the electron pairing energy, and thus indicate that spin fluctuations are intimately related to the mechanism of high $T_c$ superconductivity.

This thesis is focused on studies of substituted Hg-based superconducting copper oxides ((Hg1-xMx)Ba2Can-1CunO2n+2+δ). These compounds are promising objects of investigation, not only from a fundamental point of view but also because of their high values of superconducting transition temperature (Tc) and irreversibility field (Hirr). The first part of the thesis is devoted to optimization of the synthesis procedure for Hg-based cuprates. The influence of different parameters (T, t, p(Hg), p(O2)) on the synthesis of these compounds in sealed silica tubes was studied. Optimal conditions yielded samples containing up to 95% of HgBa2Ca2Cu3O8+δ (Hg-1223). The formation of solid solutions with the formula (Hg1-xCux)Ba2Ca2Cu3O8+δ (where x <= 0.5) was also established. Another technique was developed, using LiF as a flux, for synthesis of samples containing up to 90% of the HgBa2CaCu2O6+δ (Hg-1212) phase. The second part concerns synthesis and studies of oxyfluorides using Hg-1212 and Hg-1223 as starting materials together with XeF2 as a fluorinating agent. It was found that oxyfluorides of both phases have a parabolic dependence of Tc vs. a parameter as well as enhanced Tc values (ΔT ≈ 3-4 K) in comparison with optimally doped non-fluorinated analogues. The crystal structure of Hg-1223 oxyfluoride was studied by X-ray powder and neutron diffraction methods. It is suggested that chemical modification of the crystal structure leads to a decrease in Cu-O distance without noticeable change in Cu-O-Cu angle (in the (CuO2) layers), which may be the significant factors influencing this Tc increase. Hg-1223 oxyfluoride was also studied under high pressure for first time. It was found that this compound has a record-high Tc value (≈ 166 K) at P ≈ 23 GPa. The last part describes the investigation of substituted Hg-based superconductors in the series (Hg0.9M0.1)Ba2CuO4+δ {(Hg,M)-1201}, where M = Tl, Pb, W, Mo, Nb and V. A comprehensive study of these compounds by various methods (X-ray powder diffraction, EDX, IR-, EXAFS- and XANES -spectroscopy) indicated that the change of charge carrier doping level is a crucial factor determining the irreversibility line. (Hg0.9Mo0.1)Ba2CuO4+δ showed the most improved irreversibility line position among the (Hg,M)-1201 compounds studied in this series.

Mechanical properties of Nb-Ti composite superconducting wires were tested at room temperature. The results were analysed using simple composite theory, the rule of mixtures. The objective is to predict the mechanical properties of Nb-Ti superconducting composite wires as a function of volume ratio and geometry of the components, the composite wire size and the effect of heat treatment at final drawing wire sizes. To understand the mechanical behaviors of the Nb-Ti composite, mechanical testing of the individual composite components, Nb-Ti filament and copper matrix, was performed, and the geometry of the composite was also studied. The results indicate that for the monofilamentary composite simple composite theory with two components, Nb-Ti filament and copper matrix, can be used as the prediction of the UTS of the composite. For the multifilamentary composite three components make up the composites; a high strength Nb-Ti fiber, a low strength, high ductility bulk copper matrix and a mid-strength (between the Nb-Ti fiber's and bulk copper matrix's) interfilamentary copper matrix. After heavy cold work the UTS of Nb-Ti filaments and bulk copper matrix in the composite saturate, while the UTS of the interfilamentary copper increases as the interfilamentary spacing decreases. The UTS of the interfilamentary copper matrix as a linear function of the reciprocal of interfilamentary spacing is found. The controlling parameters in the manufacturing which determine the mechanical properties of Nb-Ti composite superconducting wires include superconductor to composite ratio, UTS of the Nb-Ti filament and copper matrix, wire final drawing size, and geometry of the composite such as size and number of the filaments, interfilamentary spacing, volume fraction of fringe and core bulk copper in multifilamentary composites. / Graduation date: 1991

The quest for obtaining higher Tc superconductivity led to the discovery of cuprates about 20 years ago. Since then, they continue to puzzle the scientific community with their bizarre properties like non-BCS superconductivity, pseudo gap, Fermi arcs, linear T resistivity etc. Since these materials show unusually high Tc, a novel mechanism is at play and strong correlations are believed to play an important role. The theme of this thesis work is to study physics of such strongly correlated systems in two dimensions at T = 0 along with development of new theoretical tools necessary for the study. The focus of the thesis is on the ground state studies of strongly correlated models like t-J and Hubbard models using variational Monte Carlo (VMC) and renormalized mean field theory (RMFT). The general method is to propose a variational wave function, motivated by the physics ideas, to be a candidate ground state of the system. Methods to efficiently evaluate the ground state energy and minimizing it with respect to the variational parameters are developed in this work. Antiferromagnetism-superconductivity competition and electron-hole asymmetry in the extended t-J model is investigated. The main result of this work is that increasing the magnitude of the next neighbor hopping (t') on hole doped side strengthen superconductivity while it stabilizes antiferromagnetism on the electron doped side. It is also shown that it is possible to characterize the T = 0 phase diagram with just one parameter called as Fermi Surface Convexity Parameter (FSCP). Next, the possibility of phase separation in the t-J model on a square lattice is investigated using local RMFT technique. It is found that for certain doping, the system phase separates into regions with antiferromagnetic and superconducting orders. Next, the role played by crystalline anisotropy of orthorhombic YBCO cuprates on their properties is examined using anisotropic tx-ty-J model and this ground state study suggests that the anisotropies seen in their properties are plausible solely due to the crystalline anisotropy. A new general method to study strongly correlated systems with singlet ground states is developed and tested in this thesis work. The last part of the thesis explores the possibility of high Tc superconductivity in graphene which is a intermediate coupling resonating valence bond (RVB) system. It is found that undoped graphene is not a superconductor, consistent with the experiments. On doping, the ground state of graphene is found to be a superconductor with “d+id” symmetry whose strength shows a dome as a function of doping which is reminiscent of RVB physics.