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This thesis makes significant advances towards an understanding of superconductivity in the cuprate family of unconventional, high-temperature superconductors. Even though the high-temperature superconductors were discovered over 35 years ago, there is not yet a general consensus on an acceptable theory of superconductivity in these materials. One of the early proposals suggested that collective magnetic excitations of the conduction electrons could lead them to form pairs, which in turn condense to form the superconducting state at a critical temperature Tc. Quantitative calculations of Tc using experimental data were, however, not available to verify the applicability of this magnetic mechanism. In this thesis, the author constructed an angle-resolved photoemission apparatus that could provide sufficiently accurate data of the electronic excitation spectra of samples in the normal state, data which was furthermore unusually devoid of any surface contamination. The author also applied the Bethe-Salpeter method to his uncommonly pristine and precise normal state data, and was able to predict the approximate superconducting transition temperatures of different samples. This rare combination of experiment with sophisticated theoretical calculations leads to the conclusion that antiferromagnetic correlations are a viable candidate for the pairing interaction in the cuprate superconductors.

Thermal properties of solids --- Theoretical spectroscopy. Spectroscopic techniques --- supergeleiding --- spectroscopie --- Superconductivity. --- Superconductors. --- Superconductors --- Spectrum analysis. --- Quantum electrodynamics. --- Spectroscopy. --- Quantum Electrodynamics, Relativistic and Many-body Calculations. --- Chemistry.

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This thesis describes in-depth studies of the remarkable electronic transport within the ultrahigh conductivity delafossite metals PtCoO_2 and PdCoO_2 using the tool of focused ion beam (FIB) microstucturing. Despite being first synthesised over 50 years ago, important questions remain regarding both the origin of the unusually high conductivity of these compounds and the consequences of their unique properties for unconventional electronic transport, such as that within the ballistic regime. The thesis explores both these areas. High-energy electron irradiation is used to examine the effects of deliberately introducing point defects into PdCoO_2 and PtCoO_2, demonstrating that the extremely low resistivity of these materials stems from an extreme purity as high as 1 defect in 120,000 atoms, rather than a novel scattering suppression mechanism. In addition, studies of the electronic transport in micron-scale squares of these metals show that their broadly hexagonal Fermi surfaces lead not only to long range ballistic behaviour but novel ballistic regime phenomena which cannot be observed in materials with a higher-symmetry Fermi surface.

Quantum mechanics. Quantumfield theory --- Statistical physics --- Thermal properties of solids --- Solid state physics --- Matter physics --- Metallurgy --- supergeleiding --- EMI (electromagnetic interference) --- materie (fysica) --- quantummechanica --- fysica --- metalen --- Condensed matter. --- Materials science --- Electronic structure. --- Quantum chemistry --- Metals. --- Superconductivity. --- Superconductors. --- Condensed Matter Physics. --- Electronic Structure Calculations. --- Metals and Alloys. --- Phase Transition and Critical Phenomena. --- Data processing. --- Computer programs. --- Copper alloys --- Nanostructured materials. --- Electric properties. --- Microstructure.