Leha O. Nonstationary processes in spatially inhomogeneous superconducting structures in electromagnetic microwave fields

Leha O.O. Nonstationary Processes in Spatially Inhomogeneous Superconducting Structures in Electromagnetic Microwave Fields. – Qualification scientific work printed as manuscript. Thesis for the degree of Doctor of Philosophy in Speciality 104 – Physics and Astronomy (Field of Knowledge 10 – Natural Sciences). — B.I. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, Kharkiv, 2024. This thesis presents experimental research on nonstationary processes in spatially inhomogeneous superconducting structures under microwave (MW) electromagnetic fields, offering new scientific and practical insights. The introduction outlines the relevance of the topic, research objectives, and methods used. The scientific novelty and practical significance of the findings are emphasized, along with details of the author's contributions and publications. Chapter 1 gives a review of literature on superconducting structures is provided, focusing on phenomena like current distribution and the formation of resistive states in superconducting films. The chapter also discusses the challenges of using superconducting resonators in electromagnetic metamaterials. The peculiarities of the work of SQUIDs are considered.Microscopic methods, particularly low-temperature laser scanning microscopy (LTLSM), are highlighted for studying these structures. The chapter 2 explores the transition of thin-film superconductors to the normal state under constant current and microwave radiation. The LTLSM method reveals that this transition involves an intermediate resistive state characterized by phase slip lines (PSLs) and normal localized domains (NLDs). The combined effect of microwave radiation and current on superconductors is found to be non-additive, with NLDs forming without altering the current-voltage (I-V) characteristics. LTLSM visualizes these phenomena, revealing details not captured by I-V characteristics alone. In particular, visualization of the formation of PSLs in a two-dimensional structure with variable current density (Dayem bridge) was performed. Chapter 3 examines The distribution of superconducting screening currents in spiral resonators is studied, crucial for developing electromagnetic metamaterials. A new LTLSM mode is introduced to visualize the phase of these currents. The research shows that at higher standing wave modes, the current distribution becomes anisotropic, indicating limitations in using such resonators in metamaterials. Chapter 4 focuses on the effect of microwave fields on the RF SQUID's amplitude-frequency characteristics. It is shown that the βL parameter, which determines the SQUID's operational mode, can be tuned using a high-frequency field. Experimental results confirm the possibility of switching the RF SQUID from a hysteretic to a non-hysteretic mode, enhancing its conversion coefficient and sensitivity. The results are scientifically and practically significant, expanding the understanding of nonstationary states in superconducting structures under microwave fields. The findings provide a basis for new research into LTLSM responses in magnetic metamaterials and improve understanding of superconducting transitions to the normal state. The research also demonstrates control over the βL parameter in RF SQUIDs, offering enhanced performance in these devices.