Critical states in type-II thin film superconductors

This thesis describes our theoretical and experimental research on the subject of the critical state model in disk-shaped type-II thin film superconductors with magnetic fields applied perpendicular to the film.; The theoretical work included analytically solving the critical state equation for a thin superconducting disk in a time-varying periodic external field assuming a constant critical current density. In the framework of Kim's critical state model, we have developed a numerical method and have solved the critical state equation for field-dependent critical current densities. The numerical results coincide with the analytical solutions in the limit of constant critical currents.; The experimental work included design and construction of the cryogenic probe as well as the measurements performed. We describe the details of the low temperature probe. Using a SQUID-based technique and the probe, we conducted magnetization measurements, studied temperature gradient effects on trapped flux, and made flux creep measurements in dc sputtered Nb, epitaxial Nb and rf sputtered NbN thin films. A great deal of effort was devoted to investigating the dc sputtered Nb films deposited by the Gravity Probe B loop coating system at Stanford since Nb is the superconducting coating of the gyroscope rotor designed for the General Relativity test. We investigated the thickness dependence of the critical current of superconducting polycrystalline Nb films. In addition, an epitaxial Nb thin film and a dc sputtered NbN thin film were used to explore a greater range of parameters. Critical currents were extracted from the magnetic hysteresis loops by applying the analytic solutions and the numerical method that we developed. Good agreement was obtained between the measured data and the theoretical fits. We discuss our analytical and numerical model, and analyze the agreement and deviations between the experimental results and the fit. Current densities and field profiles were calculated for thin superconducting disks. Experimental research was also conducted on the temperature gradient induced flux motion and the temperature gradient induced field expulsion in thin superconducting films. Thermally activated flux creep was observed and recorded for the first time in thin superconducting disks in an axial magnetic field. The thermal activation energy was deduced.