Critical Current Density and Vortex Dynamics in Pristine and Irradiated KCa(2)Fe(4)As(4)F(2)

We report the critical current density (J(c)) and vortex pinning properties in single crystals of a novel iron-based superconductor (IBS) KCa(2)Fe(4)As(4)F(2) with large J(c) in the pristine state, before and after introduction of artificial defects by swift-particle irradiation. The effects of 2.6 GeV U and 3 MeV proton irradiations in KCa(2)Fe(4)As(4)F(2) single crystals on transition temperature T(c) and J(c), including its dose dependence, are systematically studied. J(c)~8 MA/cm(2) under a self-field at 2 K in the pristine crystal is strongly enhanced up to 19.4 and 17.5 MA/cm(2) by irradiation of 2.6 GeV U-ions and 3 MeV protons, respectively. Suppression of T(c) and dose dependence of J(c) in KCa(2)Fe(4)As(4)F(2) is different from that in a representative IBS of (Ba,K)Fe(2)As(2), which can be explained by considering the presence of embedded defects in pristine KCa(2)Fe(4)As(4)F(2). The vortex dynamics in the pristine and proton irradiated KCa(2)Fe(4)As(4)F(2) single crystals are also investigated from the analyses of the field dependence of J(c) and the normalized magnetic relaxation rate. In addition to the contribution of embedded defects, weak collective pinning is considered for comprehensive analyses. Vortex dynamics in KCa(2)Fe(4)As(4)F(2) is similar to those in (Ba,K)Fe(2)As(2) to some extent, and different from that in anisotropic Li(0.8)Fe(0.2)OHFeSe. Large anisotropy, due to the presence of insulating blocking layers in KCa(2)Fe(4)As(4)F(2), which leads to much lower irreversibility field (H(irr)) compared with 122-type IBSs, strongly affect the vortex dynamics.