Flux pinning dependent quench anomaly in YBCO thin films with stabilizing Au cap layer

Abstract

High temperature superconductors (HTS) including YBa2Cu3O6+x (YBCO)-based coated conductors, have a great potential for application in innovative technologies but these materials pose considerable difficulties due to their instability at high currents and magnetic fields. This thesis examines the phenomenon of the quench anomaly found in YBCO thin films with stabilizing Au cap layer. The study is aimed at clarifying the underlying mechanism of this problem, focusing on flux pinning and vortex dynamics. Results obtained from AC susceptibility and magnetization transport measurements showed that there is enhancement of flux pinning ability in the presence of BZO doping, especially for high magnetic fields, while reducing the superconducting transition temperature slightly. Quench experiments were conducted at 65K under controlled current conditions, where a heat pulse was used to trigger quench events. One of the findings of this study is the existence of a second, delayed voltage peak (quench anomaly) after the initial quench event, once the thermal heat pulse is removed. It has been also observed that BZO doping improves the flux pinning and weakens the quench anomaly peak, whereas higher anomaly peaks observed in the undoped materials can be due to the higher Ic value. Qualitatively, the observed quench anomaly can be understood by the delay in current distribution between the superconductor and Au stabilizer, among other possible reasons. However, the quench anomaly seems to depend on the flux pinning, external magnetic field, substrate material, and stabilizing capping layer on the top. Overall, this study provides new insights into post-quench instability in HTS materials and highlights the interplay between flux pinning, thermal effects, and current dynamics. These findings are important for improving the reliability and protection strategies of superconducting devices.