High Dose Heavy-Ion Irradiation Effects on the Multiband Superconductor (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2}$

Optimal doped crystals of (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2 }$were irradiated with 1.4 GeV Pb ions to dose-matching fields ranging from 4 Tesla to 21 Tesla. Plan-view transmission electron microscopy shows creation of defects with diameters of 2 $\sim $ 5 nm. Post-irradiation characterization shows that the superconducting anisotropy is reduced to near unity, probably due to the increase in intra-band scattering. In addition, the critical current density $J_{C}$ determined from magnetization measurements shows systematic enhancement up to $\sim $5 MA/cm$^{2}$ at T=5K. We show that the decay of the critical current with magnetic field can be greatly mitigated with dense defects with approximately 20nm spacing produced by a dose matching field irradiation of 21T. Remarkably, the superconducting transition temperature remain unchanged for all matching field irradiation, suggesting that inter-band scattering due to non-magnetic impurity does not play a dominant role in pair-breaking.