Anomalous results observed in magnetization of bulk high temperature superconductors-A windfall for applications

摘要

Recent experiments on pulsed-zero field cool magnetization of bulk high J_c YBCO (YBa_2Cu_3O_(7-δ)) have shown unexpected results. For example, reproducible, non-destructive, rapid, giant field leaps (GFLs) to higher penetrated field are observed. The observations are inconsistent with the critical state model (CSM), in several aspects. Additional experiments have been pursued in an attempt to clarify the physics involved in the observed anomalies. Here, we present experimental results for the J_c dependence of the anomalous features. It is found that the sudden field increase in the GFL is a monotonically increasing function of J_c. The ratio of required pulsed field amplitude, B_(A,max), to obtain maximum trappable field, B_(T,max), which CSM predicts to be ≥2.0, gradually approaches 1.0 at high J_c. Tests using values of pulsed, applied field B_(A,max) just below the GFL exhibit two additional anomalies: (ⅰ) At high J_c, the highest trapped field is up to ～6 times lower than predicted by CSM, and (ⅱ) the measured Lorentz force as a function of J_c deviates sharply from CSM predictions. The data rule out heating effects and pinning center geometry as possible physical causes of these anomalies. A speculative cause is considered.