Scaling law for critical currents and magnetization in high temperature superconductors

Abstract

The field and temperature dependent critical current density, j c( B, T), and magnetization, M( B, T), of melt processed Bi 2Sr 2CaCu 2O 8 samples have been examined in fields up to 8 T. In addition, published data on an epitaxially grown YBa 2Cu 3O 7 film have been analysed. Both sources of information show universal field dependence of j c and M proportional to (1 − T/ T c) m exp [− B/ B o( T)] above 0.5 T. The scaling field, B o( T), contains the essential part of the temperature dependence. It is shown that neither weak links nor thermal activation of single pinning sites alone can explain this behaviour. Although it is inferred from the data at 4.2 K that a large number of pinning centres must be present in the sample, it is assumed that their efficiency is drastically reduced with rising field at elevated temperatures. It is concluded that the role of statistical summation of pinning forces to give an effective volume force should be reconsidered, and it is proposed that thermal activation reduces the number of active pinning interactions with rising temperature and field, in addition to the expected effects of flux creep.