Shytov M. Features of fluctuating conductivity and pseudogap in YBa_2Cu_3O_{7-δ} films and single crystals under the influence of external factors

The dissertation is devoted to the study of the influence of such factors as: magnetic field, irradiation with high-energy electrons, and long-term storage (aging) on the main electronic processes that form the normal and superconducting phases in high-temperature superconductors (HTSCs). of thin films and single crystals of YBa2Cu3O7- δ (hereinafter - YBCO). Chapter 1 "Main properties of studied superconductors (Literature review)" is devoted to the analysis of literature data on the topic of the dissertation. The theoretical foundations of the physics of high-temperature superconductors are considered. Concepts such as pseudogap (PG) and local pairs are considered, and different points of view on the mechanism of superconducting pairing in HTSCs are considered. Information on the temperature dependence of the excess conductivity and PG is given. The processes that occur in HTSCs when the charge carrier density decreases are considered, and the issue of anisotropy of properties and the specific behavior of local pairs in HTSCs is discussed. General information on the influence of external factors was studied in detail. Chapter 2 "Objects and methods of experimental research" covers the process of obtaining and preparing the YBCO samples for measurement. Methods of obtaining YBCO single crystals and thin films are considered in detail. A description of the design of the installation for measuring electrical resistance at low temperatures is given. Methods of measuring electrical resistance at low temperatures under the influence of a magnetic field and electron irradiation are described. In the third chapter, "The effect of a magnetic field on the properties of YBCO thin films", the evolution of the fluctuation conductivity (FLC) and the PG in a YBCO thin film under the influence of a magnetic field up to 8 T is investigated. It was found that in a magnetic field of approximately 3 T in the region of two-dimensional fluctuations there is a transition from the classical temperature dependence of the 2D Maki-Thompson FLC to the unexpected 2D Aslamazov-Larkin dependence above the temperature of the 3D-2D crossover. It was found that the PG shifted on the graph towards lower temperatures under the influence of the magnetic field, and its shape almost did not change. Chapter 4 "Influence of strong electron irradiation on the properties of YBCO single crystals" presents the results of the study of the influence of irradiation with high-energy electrons on the temperature dependence of the FLC and PG of a YBCO single crystal that does not contain twins. It is shown that a linear increase in resistivity followed by a linear decrease in the temperature of the superconducting transition is observed at all doses of irradiation. It was found that at the average value of the radiation dose, there is an increase in the opening temperature of the PG, the value of the PG at the temperature of the BEC-BCS crossover and, in general, the entire curve of the PG, which corresponds to the AG-EK transition. At an irradiation dose of 5.6⋅1019 e/cm2, FLC and PG unexpectedly exhibit curves typical of well-structured YBCO. It was found that the formation of various ensembles of defects in YBCO single crystals under the influence of high-energy electronic irradiation is a non-monotonic process and has its own specificity, and this specificity can only be detected by conducting the analysis of FLC and PG, which turned out to be much more sensitive to changes in the internal electronic subsystems. affected by induced defects In chapter 5 "Effect of long-term storage (aging) on the properties of YBCO single crystals", the effect of long-term storage of YBCO single crystals with twins for 17 years under normal conditions is investigated. The properties of the studied single crystal were studied immediately after production, after 6 and 17 years of storage. It is shown that immediately after fabrication, the sample exhibits properties typical of optimally doped YBCO single crystals containing twins and twin boundaries. After 6 years of storage, the defects that arose during aging almost completely eliminate the influence of twins and twin boundaries. As a result, the temperature behavior of FLC and PG resembles that typical for well-structured YBCO single crystals. It was found that after 17 years of storage, the large number of structural defects that appear in the sample leads to a strong degradation of the structure and charge redistribution in the crystal, which lead to a strong increase in resistance and are most likely responsible for the observed unusual temperature dependence of the FLC and PG. It was shown that the defects that occur during long-term aging significantly affect both the specific re