Vortex creep activation energies and depinning currents in CaKFe 4 As 4 and Ba 0.6 K 0.4 Fe 2 As 2 revealed by AC susceptibility measurements

Systematic ac susceptibility measurements have been carried out to study the vortex dynamics in CaKFe As and Ba K Fe As single crystals under various temperatures, dc magnetic fields, ac field frequencies and amplitudes. The field-temperature phase diagrams were shown, and the characteristics of irreversibility line were also derived. The specific expressions of activation energy on the parameters of temperature (T), current density (J), and dc magnetic field (H) are obtained according to these data. The results indicate that both superconductors have similar functional expressions of activation energy and flux pinning behaviors. Though both CaKFe As and Ba K Fe As superconductors exhibit very strong flux pinning ability, the vortex pinning potential in CaKFe As is slightly smaller than that in Ba K Fe As which may result from its distorted FeAs -tetrahedron in CaKFe As . The depinning critical current densities at the limit of low temperature and low field were also extrapolated, yielding the corresponding values of J (0) ∼ 1.0 × 10 and 2.2 × 10 A cm for CaKFe As and Ba K Fe As superconductors, respectively, which suggest potential applications.