High, magnetic field independent critical currents in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ with composite defects

We investigate the enhancement of vortex pinning by compound defects that are composed of correlated and point defects in Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ crystals with T$_{c}$ 37.5. Initial irradiation by high-energy heavy ions to a dose matching field of $B=$\textit{21}T increases vortex pinning via columnar defects with no degradation of the superconducting transition temperature. Subsequent proton irradiations further enhance the critical current \textit{Jc(H) }by suppressing the motion of vortex kinks between the columnar defects. At a temperature of 5K, we find a critical current density of 5.8 MA/cm$^{2}$ that is essentially magnetic field independent in fields up to 7 T. This work supported by the Center for Emergent~Superconductivity, an Energy Frontier Research Center funded by the U.S. D.O.E., Office of Science, Office of Basic Energy Sciences and by the D.O.E, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. The operation of the ATLAS facility was supported by the U.S. D.O.E., Office of Nuclear Physics, under Contract No. DE-AC02-06CH11357. The work in China was supported by the NSF of China, the MOST of China (2011CBA00102 and 2012CB821403) and PAPD.