Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10^-4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high-T_c superconducting materials. The low-field, step-like drop in the critical current density J_c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current (V-I characteristic, represented by the transition parameter n (i.e., V∝Iⁿ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J_c(B) characteristic.