CRITICAL FIELDS AND CURRENT DENSITIES IN YBa₂Cu₃O7-δ

Abstract
An extensive investigation of the magnetic and transport properties of the new high-T_c superconductors is reported. The magnetic cycle of granular materials is characterized by a low field (H < H_c1) structure which is shape dependent whereas the high field magnetic cycle is shape independent. On the contrary, single crystals show no low-H structure but a shape dependent magnetization at high field. The magnetic critical-current density deduced from the high-field cycle is about the same (~ 10⁶ A/cm² at 4.2 K) in the two materials. The transport current (~ 10³ A/cm²at 4.2 K) is related to the low-H magnetic cycle by a Silsbee-like rule and is governed by the Josephson-junctions between the grains. The above data are interpreted in terms of a simple model based on the Ferrell-Prange and Ambegaokar-Baratoff equations and on the critical state concept.