Tuning the Lower Limit to the Vortex Creep Rate in Iron-based Superconductors

In type II superconductors, the critical current density J_c depends on the dissipative dynamics of vortices, magnetic flux lines that penetrate the materials upon exposure to magnetic fields. Vortex motion, which is dissipative, can be thermally activated (vortex creep) or driven by current-induced forces. The standard approach to boost the critical current density and slow creep is to add defects to the material, which creates energetically favorable positions for vortices to localize, thereby counteracting vortex motion. However, when optimized, this approach can only yield a maximum J_c of ~30% J_d, where J_d is the depairing current, and a minimum creep rate of S~G_i^1/2(T/T_c), where G_i is the Ginzburg parameter, T is temperature, and T_c is the critical temperature. In this study, we tune the theoretical limits by doping iron-based superconductors to increase the superfluid density, which reduces the penetration depth λ thereby increasing J_d~1/(ξλ²) and decreasing G_i~T_c²λ⁴/ξ². We find that this approach does indeed result in dramatic increases in J_c and reductions in S. Our results suggest that combining this approach with modifications of the material defect landscape could lead to unprecedentedly high J_c and slow creep S in superconductors.