Quadrupling the depairing current density in the iron-based superconductor SmFeAsOH.

Iron-based 1111-type superconductors display high critical temperatures and relatively high critical current densities J. The typical approach to increasing J is to introduce defects to control dissipative vortex motion. However, when optimized, this approach is theoretically predicted to be limited to achieving a maximum J of only ∼30% of the depairing current density J, which depends on the coherence length and the penetration depth. Here we dramatically boost J in SmFeAsOH films using a thermodynamic approach aimed at increasing J 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 J reaches 415 MA cm, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high J values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.