Designing high-performance superconductors with nanoparticle inclusions: comparisons to strong pinning theory

Abstract

One of the most promising routes for achieving unprecedentedly high critical currents in superconductors is to incorporate dispersed, non-superconducting nanoparticles to control the dissipative motion of vortices. However, these inclusions reduce the overall superconducting volume and can strain the interlaying superconducting matrix, which can detrimentally reduce T c . Consequently, an optimal balance must be achieved between the nanoparticle density n p and size d. Determining this balance requires garnering a better understanding of vortex-nanoparticle interactions, described by strong pinning theory. Here, we map the dependence of the critical current on nanoparticle size and density in (Y 0.77 ,Gd 0.23 )Ba 2 Cu 3 O 7−δ films in magnetic fields up to 35 T, and compare the trends to recent results from timedependent Ginzburg-Landau simulations. We identify consistencies between the field-dependent critical current J c (B) and expectations from strong pinning theory. Specifically, we find that that J c ∝ B −α , where α decreases from 0.66 to 0.2 with increasing density of nanoparticles and increases roughly linearly with nanoparticle size d/ξ (normalized to the coherence length). At high fields, the critical current decays faster (∼ B −1 ), suggestive that each nanoparticle has captured a vortex. When nanoparticles capture more than one vortex, a small, high-field peak is expected in J c (B). Due to a spread in defect sizes, this novel peak effect remains unresolved here. Lastly, we reveal that the dependence of the vortex creep rate S on nanoparticle size and density roughly mirrors that of α, and compare our results to low-T nonlinearities in S(T ) that are predicted by strong pinning theory.