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Abstract
Transport characteristics of nano-sized superconducting strips and bridges are determined by an intricate interplay of surface and bulk pinning. In the limiting case of a very narrow bridge, the critical current is mostly defined by its surface barrier, while in the opposite case of very wide strips it is dominated by its bulk pinning properties. Here we present a detailed study of the intermediate regime, where the critical current is determined, both, by randomly placed pinning centres and by the Bean-Livingston barrier at the edge of the superconducting strip in an external magnetic field. We use the time-dependent Ginzburg-Landau equations to describe the vortex dynamics and current distribution in the critical regime. Our studies reveal that while the bulk defects arrest vortex motion away from the edges, defects in their close vicinity promote vortex penetration, thus suppressing the critical current. We determine the spatial distribution of the defects optimizing the critical current and find that it is in general non-uniform and asymmetric: the barrier at the vortex-exit edge influence the critical current much stronger than the vortex-entrance edge. Furthermore, this optimized defect distribution has a more than 30% higher critical current density than a homogeneously disorder superconducting film.
Highlights
Immobilizing magnetic vortices and preventing dissipation under applied currents is one of the major objectives for realizing applications of type-II superconductivity[1,2,3,4]
It was observed that the introduction of point-like or cylindrical defects near the surface can be detrimental to the effectiveness of surface barriers[18,19] since they promote easier vortex penetration across the surface[20]
In the case of narrow strips with widths on the order of the superconducting coherence length, the critical current is mostly defined by its surface barrier and phase slips across the strip are important[25,26], while for very wide strips, the critical current is dominated by its bulk pinning properties
Summary
ForW 1, the critical current Ic saturates at some certain value defined by the depinning forces of the two barriers and depends on the magnetic field. In this case, the critical current associated with pinning vortices at non-superconducting defects depends on the defect properties (shape, size, concentration) and on the field strength (vortex density). We design the pinning configuration within our model system with finite W in the following way: (i) the density of the non-superconducting columnar defects far away from the edges is the same as in the bulk case corresponding to the maximum possible critical current; (ii) the density of non-superconducting defects near edges is linearly modulated towards the edges. On the opposite side of the sample ρi(y) changes from f to fout at distance lout
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