Resistive layers formation during the superconductor-normal transition of high-Tc superconductors

The formation of layers during the resistive transition of granular high-T_c superconductors, characterized by either weak (YBCO-like) or strong (MgB₂-like) links, occurs through a series of avalanche-type current density rearrangements. These resistive layers cross the whole specimen approximately orthogonal to the current density direction, and are due to the simultaneous transition of a large number of weak-links or grains. In the present work, strongly and weakly linked networks of nonlinear resistors, with Josephson junction characteristics are considered. It is shown that the exact solution of the Kirchhoff equations yields the subsequent formation of resistive layers within the superconductive matrix as temperature increases. Furthermore, the resistive layer formation process is related to the voltage noise observed at the transition. At the end of the transition, as experimentally found, the layers mix-up, the step amplitude decreases and the resistance curve smoothes. The approach can be extended to networks with arbitrary size and, thus, to real specimens.