Limit to Self-Field Critical Current Density in Thin-Film, Type-II Superconductors

In the last decade, the self-field critical current density J_c(s.f.) in Type-II superconductors has been considered fundamentally limited by a Silsbee-like criterion of J_c(s.f.) = H_c1/λ. We show that this universal limit to self-field critical current density J_c(s.f.) is not universally valid. We present several examples for this in YBa₂Cu₃O_7-δ-type and REBa₂Cu₃O_7-δ thin films and one for Nb thin films and show that calculated J_c(s.f.) using the Silsbee-like criterion using thermodynamic parameters has been substantially exceeded experimentally. We also show that J_c(s.f.) can be significantly improved by incorporation of artificial pinning centers (APCs), further implying that no such universal limit to J_c(s.f.) can exist because such an upper bound, J_c(s.f.) would have to be independent of APCs. These findings call for a revision of the accepted understanding of current-carrying limits in Type-II superconductors and reveal substantial potential for improving J_c in REBCO-based coated conductors through optimization of APCs for large-scale applications, including commercial nuclear fusion.