Multiple superconducting transition in ceramic YBa2Cu3O7- delta.

Abstract

The temperature derivative of the resistance is utilized to investigate current density effects on the resistive superconducting transition in ceramic $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. In this analysis the transition is presented as a peak with definite midpoint temperature and width. The zero-resistance and onset temperatures are also determined unambiguously. Multiple transition regimes were observed with increasing current density. A tail regime below the major transition, which is responsible for the low current densities reported by many laboratories, is shown to broaden markedly with current density. The temperature dependence of the critical current in the tail regime is close to linear above 77 K. The tail regime, that is also very sensitive to preparation and to aging through environmental attack, originates probably from Josephson-coupled, randomly oriented grain boundaries. The main transition regime, on the other hand, is quite inert to preparation and environmental contamination; it has a much higher critical current and a markedly different (i.e., nonlinear) temperature dependence of the critical current near the transition. It is likely that this component of the critical current, as observed in the main transition regime, represents the intragranular critical current in ceramic superconductors.