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 J_c. The typical approach to increasing J_c 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_c of only ∼30% of the depairing current density J_d, which depends on the coherence length and the penetration depth. Here we dramatically boost J_c in SmFeAsO_1-xH_x films using a thermodynamic approach aimed at increasing J_d 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_d reaches 415 MA cm^-2, a value comparable to cuprates. Finally, by introducing defects using proton irradiation, we obtain high J_c values in fields up to 25 T. We apply this method to other iron-based superconductors and achieve a similar enhancement of current densities.