Single crystalline FeTe0.61Se0.39 with a sharp superconducting transition at Tc ∼ 14 K is synthesized via slow furnace cooling followed by low-temperature annealing. The effect of annealing on the chemical and superconducting inhomogeneities is carefully characterized. We also report resistivity, magnetization, and magneto-optical images of this crystal. Based on the Bean model, critical current density is estimated to exceed 1 × 10 5 A/cm 2 below 5 K under zero field. Weak fish-tail effect is identified at lower temperatures.

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.

We present the superconducting properties of single-crystalline Ba(Fe 0:9 Co 0:1 ) 2 As 2 by measuring its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal a fish-tail hysteresis loop at high temperatures and a relatively high critical current density above J c ¼ 10 5 A/cm 2 at low temperatures. The upper critical field determined by resistive transition is anisotropic with an anisotropic parameter of $3:5. Hall effect measurements indicate that Ba(Fe 0:9 Co 0:1 ) 2 As 2 is a multiband system and that the mobility of electrons is dominant. Magneto-optical imaging reveals a prominent Bean-like penetration of vortices, although there is a slight inhomogeneity in a sample. Moreover, we observe distinct superconductivity above 25 K, which leads us to speculate that a higher transition temperature can be realized by fine-tuning the Co-doping level.Since the discovery of the high-T c iron-based oxypnictide superconductor LaFeAsO 1Àx F x with T c $ 26 K, 1) other ironbased superconductors have been sought for to obtain a higher transition temperature. In rare-earth-substituted iron oxypnictides RFeAsO 1Àx F x (R = rare earth), transition temperature has been increased up to 55 K. 2) In these iron oxypnictides, superconductivity is induced by introducing electrons in the (FeAs) À layers by substituting F for O. Following these discoveries, oxygen-free iron-arsenide AFe 2 As 2 (A ¼ Ba, Sr, Ca) was discovered. These materials show superconductivity with the substitution of alkali metals, such as Na, K, and Cs, for A resulting in the introduction of holes in the (FeAs) À layers. 3-6) In hole-doped oxygen-free iron arsenides, transition temperature is increased to $38 K. 3,4) On the other hand, very recent studies revealed that the electron doping by the substitution of Co or Ni, which have one or two excess d electrons compared with Fe in conducting layers, induces superconductivity in oxygen-free iron arsenide. 7,8) In fact, nuclear magnetic resonance measurements have revealed that Co atoms donate electrons without generating localized moments. 9) Although the highest transition temperature in electrondoped BaFe 2 As 2 is reported to be $23 K, 10) which is lower than that in hole-doped BaFe 2 As 2 , the fact that the substitution of transition metals leads to the induction of superconductivity contrasts strongly with a drastic suppression of T c in cuprates. 11) However, detailed study of transition temperature as a function of Co-doping level is limited. 10) It is an open question whether the highest T c in electron-doped BaFe 2 As 2 can be increased further.In this study, we prepare a single-crystalline sample of Ba(Fe 0:9 Co 0:1 ) 2 As 2 and present its superconducting properties by studying its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. We address the possibility of further enhancing transition temperature by fine-tuning Co-doping level.Single-crystalline samples with a nominal comp...