Doping dependence of the pinning efficiency in K-doped Ba122 single crystals prior to and after fast neutron irradiation

A sharp peak was observed in the doping dependence of the critical current density, J c , in potassium doped Ba122 single crystals. This behavior is in contrast to the doping dependence of the transition temperature, T c , which varies much more smoothly around its maximum. We performed fast neutron irradiation on the crystals in order to find out whether the J c peak results from intrinsic properties or the particular defect landscape. Fast neutrons are known to introduce defects up to a size of a few nanometers, which have proven to be more efficient for flux pinning than the crystallographic defects in the pristine crystals. We find that the peak in J c shifts to higher doping levels after the irradiation, broadens, and roughly follows the shape of the T c curve. Moreover, a power law between J c and T c is observed in the irradiated crystals, which can be explained by relations between fundamental parameters and T c observed previously in iron-based superconductors. This power law does not hold for the pristine crystals which indicates that the doping dependence of J c results from an enhanced pinning efficiency in the under-doped area of the phase diagram.