Quadrupling the depairing current density in the iron-based superconductor SmFeAsO_1-xH_x

Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities (Jc). The typical approach to increasing Jc is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum Jc of only ~30% of the depairing current density (Jd), which depends on the coherence length and the penetration depth. Here we dramatically boost Jc in SmFeAsO1−xHx films using a thermodynamic approach aimed at increasing Jd and incorporating vortex pinning centres. Specifically, we reduce the penetration depth, coherence length and critical field anisotropy by increasing the carrier density through high electron doping using H substitution. Remarkably, the quadrupled Jc reaches 415 MA cm−2, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high Jc values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.