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This book studies the structural, magnetic and electronic properties of, as well as magnetic excitations in, high-temperature BaFe2-xNixAs2 superconductors using neutron diffraction and neutron spectroscopic methods. It describes the precise determination of the phase diagram of BaFe2-xNixAs2, which demonstrates strong magnetoelastic coupling and avoided quantum criticality driven by short-range incommensurate antiferromagnetic order, showing cluster spin glass behavior. It also identifies strong nematic spin correlations in the tetragonal state of uniaxial strained BaFe2-xNixAs2. The nematic correlations have similar temperature and doping dependence as resistivity anisotropy in detwinned samples, which suggests that they are intimately connected. Lastly, it investigates doping evolution of magnetic excitations in overdoped BaFe2-xNixAs2 and discusses the links with superconductivity. This book includes detailed neutron scattering results on BaFe2-xNixAs2 and an introduction to neutron scattering techniques, making it a useful guide for readers pursuing related research.

Phase diagrams. --- Superconductivity. --- Iron-based superconductors. --- Iron oxypnictide superconductors --- Iron oxypnictides --- Iron pnictide superconductors --- Iron pnictides --- Oxypnictide superconductors --- Oxypnictides --- Physics. --- Superconductors. --- Magnetism. --- Magnetic materials. --- Physical measurements. --- Measurement. --- Strongly Correlated Systems, Superconductivity. --- Measurement Science and Instrumentation. --- Magnetism, Magnetic Materials. --- Electric conductivity --- Critical currents --- Superfluidity --- High temperature superconductors --- Phase rule and equilibrium --- Physical metallurgy --- Mathematical physics --- Physics --- Electricity --- Magnetics --- Measurement   . --- Materials --- Measuring --- Mensuration --- Mathematics --- Technology --- Metrology --- Physical measurements --- Measurements, Physical --- Measurement --- Superconducting materials --- Superconductive devices --- Cryoelectronics --- Electronics --- Solid state electronics

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This thesis combines highly accurate optical spectroscopy data on the recently discovered iron-based high-temperature superconductors with an incisive theoretical analysis. Three outstanding results are reported: (1) The superconductivity-induced modification of the far-infrared conductivity of an iron arsenide with minimal chemical disorder is quantitatively described by means of a strong-coupling theory for spin fluctuation mediated Cooper pairing. The formalism developed in this thesis also describes prior spectroscopic data on more disordered compounds. (2) The same materials exhibit a sharp superconductivity-induced anomaly for photon energies around 2.5 eV, two orders of magnitude larger than the superconducting energy gap. The author provides a qualitative interpretation of this unprecedented observation, which is based on the multiband nature of the superconducting state. (3) The thesis also develops a comprehensive description of a superconducting, yet optically transparent iron chalcogenide compound. The author shows that this highly unusual behavior can be explained as a result of the nanoscopic coexistence of insulating and superconducting phases, and he uses a combination of two complementary experimental methods - scanning near-field optical microscopy and low-energy muon spin rotation - to directly image the phase coexistence and quantitatively determine the phase composition. These data have important implications for the interpretation of data from other experimental probes.

High temperature superconducters. --- Iron-based superconductors. --- Iron oxypnictide superconductors --- Iron oxypnictides --- Iron pnictide superconductors --- Iron pnictides --- Oxypnictide superconductors --- Oxypnictides --- Physics. --- Superconductivity. --- Superconductors. --- Nanoscale science. --- Nanoscience. --- Nanostructures. --- Spectroscopy. --- Microscopy. --- Optical materials. --- Electronic materials. --- Nanotechnology. --- Strongly Correlated Systems, Superconductivity. --- Spectroscopy and Microscopy. --- Optical and Electronic Materials. --- Nanoscale Science and Technology. --- High temperature superconductors --- Molecular technology --- Nanoscale technology --- High technology --- Optics --- Materials --- Nanoscience --- Physics --- Nano science --- Nanoscale science --- Nanosciences --- Science --- Electronic materials --- Analysis, Microscopic --- Light microscopy --- Micrographic analysis --- Microscope and microscopy --- Microscopic analysis --- Optical microscopy --- Analysis, Spectrum --- Spectra --- Spectrochemical analysis --- Spectrochemistry --- Spectroscopy --- Chemistry, Analytic --- Interferometry --- Radiation --- Wave-motion, Theory of --- Absorption spectra --- Light --- Spectroscope --- Superconducting materials --- Superconductive devices --- Cryoelectronics --- Electronics --- Solid state electronics --- Electric conductivity --- Critical currents --- Superfluidity --- Qualitative --- Spectrometry --- Analytical chemistry

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This thesis presents various characteristics of 122-type iron pnictide (FeSC) such as crystal and electronic structure, carrier-doping effect, and impurity-scattering effect, using transport, magnetization, specific heat, single-crystal X-ray diffraction, and optical spectral measurements. Most notably the measurement on the magnetic fluctuation in the material successfully explains already known unusual electronic properties, i.e., superconducting gap symmetry, anisotropy of in-plane resistivity in layered structure, and charge dynamics; and comparing them with those of normal phase, the controversial problems in FeSCs are eventually settled. The thesis provides broad coverage of the physics of FeSCs both in the normal and superconducting phase, and readers therefore benefit from the efficient up-to-date study of FeSCs in this thesis. An additional attraction is the detailed description of the experimental result critical for the controversial problems remaining since the discovery of FeSC in 2008, which helps readers follow up recent developments in superconductor research.

Physics. --- Solid state physics. --- Superconductivity. --- Superconductors. --- Magnetism. --- Magnetic materials. --- Spectroscopy. --- Microscopy. --- Optical materials. --- Electronic materials. --- Strongly Correlated Systems, Superconductivity. --- Optical and Electronic Materials. --- Spectroscopy and Microscopy. --- Solid State Physics. --- Magnetism, Magnetic Materials. --- Electronic materials --- Optics --- Analysis, Microscopic --- Light microscopy --- Micrographic analysis --- Microscope and microscopy --- Microscopic analysis --- Optical microscopy --- Analysis, Spectrum --- Spectra --- Spectrochemical analysis --- Spectrochemistry --- Spectroscopy --- Superconducting materials --- Superconductive devices --- Natural philosophy --- Philosophy, Natural --- Materials --- Mathematical physics --- Physics --- Electricity --- Magnetics --- Iron-based superconductors. --- Iron oxypnictide superconductors --- Iron oxypnictides --- Iron pnictide superconductors --- Iron pnictides --- Oxypnictide superconductors --- Oxypnictides --- High temperature superconductors --- Cryoelectronics --- Electronics --- Solid state electronics --- Solids --- Spectrometry --- Chemistry, Analytic --- Interferometry --- Radiation --- Wave-motion, Theory of --- Absorption spectra --- Light --- Spectroscope --- Electric conductivity --- Critical currents --- Superfluidity --- Qualitative --- Analytical chemistry