Vortex edge barriers and critical current density in granular superconductors

Abstract

Type-II superconductors, even without bulk pinning and in the absence of a macroscopic BeanLivingston surface barrier for vortex penetration, can exhibit barriers for flux penetration. We investigated some new edge barriers based on the laminar model, that are governed by the anisotropy ratio, grain coupling strength, and grain size. Expressions describing the pinning potential U p and critical current density J c are derived for magnetic fields near H c1 . It is shown that edge barrier effects play a substantial role in a realistic description of the critical current density for both MgB 2 and high-T c superconductors.

Grain boundary effects on transport properties in strongly coupled superconductors: A Monte Carlo study

“…It is easily seen that if a variation of η takes place, then the J c (H) in the bulk is almost unchanged in the case of a large grain-coupling parameter (MgB 2 ), while the J c value increases when the parameter is small (for instance, Y -and Bi-based superconductors). This tendency is similar to that in a single grain [10,11].…”

“…Oppositely, in the bulk, the increase of J c may be entirely due to the decrease of λ eff . The above results mean that one cannot interpret the grain boundary effects to transport current in the real strong-coupling superconducting sample taking into account only single-grain transport properties [10,11].…”

“…However, it is easily seen that although in a grain the critical current J G c decreases as the grain-coupling parameter σ increases, the critical current in the bulk J c increases as that parameter increases. In the grain, this reduction in J G c may be entirely due to the decreasing of pinning potential U p [10,11]. Oppositely, in the bulk, the increase of J c may be entirely due to the decrease of λ eff .…”

“…It seems reasonable that the MgB 2 superconductors can be modeled as regular arrays of Josephson junction when the effects due to positional disorder are disregarded. The superconductor-to-normal transition of each grain can be induced by the variation of magnetic field and grain size [11], at first. To describe this situation, the correlation effects, and percolation theory cannot be applied straightforwardly.…”

“…For the vortex-laminar model, the J ij c value defined relative to the pinning potential of the ij-grain U ij p , as noted in refs. [10,11], is…”