We report the first realization of columnar defects in Co-doped BaFe2As2 single crystals by heavyiron irradiation. The columnar defects are confirmed by transmission electron microscopy and their density is about 40 % of the irradiation dose. Magneto-optical imaging and bulk magnetization measurements reveal that the critical current density is strongly enhanced in the irradiated region. We also find that vortex creep rates are strongly suppressed by the columnar defects. We compare the effect of heavy-ion irradiation into Co-doped BaFe2As2 and cuprate superconductors. PACS numbers: 74.25.Qt, 74.25.Sv, 74.70.Dd A limitation on the technological advances of hightemperature superconductors comes from the intrinsically low critical current density J c . Recently discovered iron-based superconductors [1] also face the same problem. While the transition temperature T c is increased up to ∼ 55 K in rare-earth-based iron-oxyarsenides within a short period of time [2], J c at low temperatures is still low [3,4,5]. Although the transition temperature of iron-pnictide superconductors is still lower than cuprate superconductors, introduction of pinning centers can enhance the critical current density and make this system more attractive for practical applications. It is well known that a most efficient way to improve the critical current density is to pin vortices with columnar defects created by swift particle irradiation. In high temperature superconductors, columnar defects enhances J c dramatically [6,7].Intermetallic iron-arsenides Ba(Fe 1−x Co x ) 2 As 2 with T c ∼ 24 K is readily available in large single crystalline form [8] and its J c reashes 10 6 A/cm 2 at T = 2 K, which is potentially attractive for technological applications [3,4,5]. We expect that J c in Ba(Fe 1−x Co x ) 2 As 2 could be enhanced by introducing columnar defects that can pin vortices. However, it is well known that irradiationinduced defects strongly depend on various parameters such as ion energy, stopping power of incident ions, thermal conductivity and perfection of the target crystal, etc.[9] Since there are so many influencing factors, it is still an open question whether irradiation damage can be introduced in iron-arsenide superconductors. In this paper, we report the first attempt to create columnar defects by heavy-iron irradiation into Co-doped BaFe 2 As 2 single crystals. Columnar tracks with diameters of ∼ 2-5 nm and about 40% of nominal ion dose are clearly seen in scanning transmission electron microscopy (STEM) images. Magneto-optical images and bulk magnetization measurements reveal a strong enhancement of J c in the irradiated region. Columnar defects also suppress the relaxation of magnetization consistent with the enhancement of pinning capability of vortices. Effect of columnar defects on the vortex dynamics is compared between Codoped BaFe 2 As 2 and high temperature superconductors.Single crystalline samples of Ba(Fe 0.93 Co 0.07 ) 2 As 2 were grown by FeAs/CoAs self-flux method [5]. Co concentration was determined by ED...

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Various kinds of energetic particles are irradiated into iron-based superconductors, and their effects on the critical current density (J c ) and vortex dynamics have been systematically studied. It is found that J c is enhanced and vortex dynamics is strongly suppressed by energetic particles having a sufficient energy deposition rate, similar to the case of high temperature cuprate superconductors. The enhancement of J c , in general, persists up to much higher irradiation doses than in cuprates. However, details of the effect of irradiation depend on the kind of ion species and their energies. Even with the same ions and energies, the effect is not universal for different kinds of iron-based superconductors. The correlated nature of defects created by heavy-ion irradiation is confirmed by the angular dependence of irreversible magnetization.