Barrier or easy-flow channel: The role of grain boundary acting on vortex motion in type-II superconductors*

Abstract

Grain boundaries (GBs), as extremely anisotropic pinning defects, have a strong impact on vortex motion in type-II superconductors, and further on the macro level dominates the superconductivity for example the critical current density. Many previous studies indicated that mostly GB plays the role of a strong barrier for vortex motion, while an easy-flow channel just under some certain conditions. In order to thoroughly make clear of the questions of what is exactly the role of GB on vortex motion and how it works, in this article we developed a large scale molecular dynamic model and revealed the action of GB on vortex motion in type-II superconductors. The most significant finding is that the role of GB on vortex motion can be changeable from a barrier to an easy-flow channel, and which is intrinsically determined by the competition effect correlated with its action on vortex between in the GB and no-GB regions. Such the competition effect essentially depends on the attributes of both the GB (described by the GB strength and angle θ) and no-GB pining regions (by the relative disorder strength α p/α v). Specifically, for a YBa2Cu3O7 – x (YBCO) sample, to obtain a clear knowledge of vortex motion in GB region, we visualized the three typical trajectories of vortices during the three vortex movement stages. Further, in order to understand how GB results in the macro current-carrying property, corresponding to the current–voltage relation of the YBCO conductor, we obtained the average velocity vy of vortices varying with their driving force, which is nearly identical with the previous observations.

Role of grain boundary networks in vortex motion in superconducting films

“…The geometric structure we simulate is an infinite superconducting slab containing the GB networks with random angles, which belongs to the category of two-dimensional (2D) periodic system. According to previous researches, [26] the GBs are modelled as the potential wells, because the behavior of the vortices in the vicinity of the GB is similar to the motion of charge carriers caused by the electric field. We assume that the GB is composed of a series of dislocations (red dots) distributed along the GB and the local electric fields applied to the specific areas, normal to the direction of the GB as shown in Fig.…”

“…The related parameters E (the electric field intensity) and q (charge of the vortices) are tunable parameters. The pinning potential of the special pins along the GB is determined by [26] U vs (r…”

“…[22][23][24][25] As is well known, the role of the GB which can be changed from a barrier to an easy-flow channel is intrinsically determined by the competitive effect associated with the action on vortex between in the GB and intragranular region, and also affected by the angle between the grain boundary and the applied current based on previous work. [26] The investigations on random GB networks with complex orientations which can model polycrystalline materials also play a crucial role in gaining a deeper knowledge about grain boundary mechanism on vortex motion. [27,28] In the present work, the GB networks are generated in the coordinates of the nuclei of crystals by using Voronoi tessellation.…”

Improving critical current density of Nb3Sn by optimizing pinning potential of grain boundary and grain size

“…Later, for a system with small size, it was found that the critical current density is not sensitive to grain size until the grain size is sufficiently small in polycrystalline superconductors [37], which is not consistent with experiments of Nb 3 Sn. It was also found that GBs not only acts as the pinning centers for vortices, but also provides an easy-flow channel for vortex motion [38,39], which depends on the GB angle [40][41][42]. However, the previous theoretical works mainly focus on one GB or very small amount of GBs.…”