The dissertation is devoted to the investigation of electrical transport phenomena, charge carrier scattering mechanisms, and magnetic flux dynamics in YBaCuO high-temperature superconducting cuprates under extreme external conditions, including high hydrostatic pressure, magnetic field, and doping with praseodymium and titanium. The relevance of the study is determined by the need to improve the understanding of the physical mechanisms governing the transport properties of high-temperature superconductors and to establish the influence of structural defects and external factors on their superconducting characteristics.
The object of the research is charge transport and magnetic flux dynamics in single-crystalline and ceramic compounds of the YBa₂Cu₃O₇−δ and Y₁₋zPrzBa₂Cu₃O₇−δ systems. The subject of the research comprises electrical transport processes, charge carrier scattering mechanisms, fluctuation conductivity, electrical resistivity anisotropy, flux pinning, and the influence of controlled structural defects on the physical properties of these materials.
To achieve the research objectives, high-quality single crystals and ceramic samples with controlled defect morphology were prepared. Structural characterization was performed using X-ray diffraction, optical microscopy, and electron microscopy. Electrical transport properties were investigated by the standard four-probe technique over a wide range of temperatures, magnetic fields, and hydrostatic pressures.
The influence of praseodymium substitution on the electrical conductivity anisotropy and charge transport mechanisms in Y₁₋zPrzBa₂Cu₃O₇−δ single crystals was established. It was shown that increasing the praseodymium content results in charge carrier localization, promotes the metal–insulator transition, and increases the semiconducting contribution to electrical conductivity. The temperature dependence of resistivity anisotropy is satisfactorily described by the variable-range hopping model, indicating the significant role of Coulomb interactions and structural anisotropy in charge transport processes.
The effect of high hydrostatic pressure on the electrical resistivity and fluctuation conductivity of Y₀.₆₆Pr₀.₃₄Ba₂Cu₃O₇−δ single crystals was investigated. The temperature dependence of the normal-state resistivity is well described by the Bloch–Grüneisen model, which takes into account charge carrier scattering by phonons and impurities. The pseudogap opening temperature was determined, and the evolution of fluctuation conductivity was analyzed within the Lawrence–Doniach model. The obtained results demonstrate that hydrostatic pressure improves the structural perfection of the crystals and enhances their superconducting characteristics.
Special attention was paid to the influence of titanium doping on magnetoresistance and flux pinning in YBaCuO superconductors. Titanium incorporation was found to modify the resistive superconducting transition, suppress the additional paracoherent transition, and alter the vortex matter phase state. These effects are associated with the formation of secondary-phase inclusions acting as effective bulk flux-pinning centers.
The scientific novelty of the dissertation consists in establishing the regularities of the influence of praseodymium and titanium doping, as well as high hydrostatic pressure, on electrical transport mechanisms, fluctuation conductivity, and magnetic flux dynamics in YBaCuO high-temperature superconducting cuprates. The obtained results expand the current understanding of charge transport processes in anisotropic high-temperature superconductors and may be applied in the development of advanced superconducting materials with improved transport and current-carrying characteristics for cryogenic engineering, superconducting electronics, and power applications.
Keywords: high-temperature superconductors, YBaCuO, single crystals, electrical transport, conductivity anisotropy, fluctuation conductivity, pseudogap, flux pinning, magnetoresistance, hydrostatic pressure, praseodymium, titanium.
