Petrenko E. Study of effects affecting fluctuation conductivity and the formation of a pseudogap state in cuprate HTSCs and magnetic superconductors

The dissertation is devoted to the search and study of information about fundamental electronic processes that form the normal and superconducting (SC) phase in high-temperature superconductors (HTSCs), under the influence of various effects in order to obtain information about the physical nature of the interaction between superconductivity and magnetism. In this regard, for the first time the information about the effect of annealing has been obtained in the atmosphere of oxygen, and then at room temperature on the excess conductivity σ΄(T) and the pseudogap Δ*(T) of the untwinned YBa2Cu3O7-δ single crystal with small deviation from oxygen stoichiometry. For the first time, comparing experimental data with the Peters-Bauer (PB) theory, the density of local pairs <n↑n↓> ≈ 0.3 near Tc was estimated, which is probably a universal value for HTSCs. It has been shown for the first time that the change under pressure of almost all measured parameters in Y0.95Pr0.05Ba2Cu3O7-δ single crystals is of a threshold nature, starting from P ~ 1 GPa. It has been shown for the first time that, in FeSe samples, the range of the superconducting fluctuations extends to temperatures more than 2 times higher than Tc. For the first time, the temperature dependences of the Δ*(Т) parameter, which in cuprates is associated with a pseudogap, have been obtained for FeSe. It has been shown that the density of local pairs near Tc, determined within the PB theory, depends significantly on the various defective ensembles that arise in the manufacture of individual samples.