Effect of flux avalanches on activation energy in type-II superconductors: Evidence for self-organized criticality

Abstract

The behavior of thermally activated flux avalanches is studied for both high-${\mathit{T}}_{\mathit{c}}$ superconductors (single crystals of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{\mathit{x}}$) and a conventional type-II superconductor (${\mathrm{Nb}}_{3}$Sn multifilamentary wire). By combining two different mechanisms, namely, thermally activated and avalanche flux motion, we have developed a U(j) relationship in a large regime of driving force for type-II superconductors. As a result of flux avalanches, the flux-motion activation energy is considerably reduced. We have found a dynamic crossover at which the flux-motion characteristic changes from spatially and temporally correlated flux avalanches to slow creep.