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

In the last decade, the self-field critical current density Jc(s.f.) in Type-II superconductors has been considered fundamentally limited by a Silsbee-like criterion of Jc(s.f.) = Hc1/λ. We show that this universal limit to self-field critical current density Jc(s.f.) is not universally valid. We present several examples for this in YBa2Cu3O7−δ-type and REBa2Cu3O7−δ thin films and one for Nb thin films and show that calculated Jc(s.f.) using the Silsbee-like criterion using thermodynamic parameters has been substantially exceeded experimentally. We also show that Jc(s.f.) can be significantly improved by incorporation of artificial pinning centers (APCs), further implying that no such universal limit to Jc(s.f.) can exist because such an upper bound, Jc(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 Jc in REBCO-based coated conductors through optimization of APCs for large-scale applications, including commercial nuclear fusion.