Superconductivity at T_c ~ 14 K in Single Crystalline FeTe0.61_{0.61}Se0.39_{0.39}

Single crystalline FeTe$_{0.61}$Se$_{0.39}$ with a sharp superconducting transition at $T_{\textrm{c}} \sim$ 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 $1 \times 10^5$ A/cm$^2$ below 5 K under zero field. Weak fish-tail effect is identified at lower temperatures.Comment: 4 pages, 4 figure

Enhancement of critical current density and vortex activation energy in proton-irradiated Co-doped BaFe2As2

The effect of proton irradiation in Ba(Fe0.93Co0.07)2As2 single crystals is reported. We analyze temperature dependence of current density and normalized flux relaxation rate in the framework of collective creep model. Glassy exponent and barrier height for flux creep are directly determined by Maley's method. Our model functions for barrier height and critical current density in the absence of flux creep are explained by the superposition of \deltaTc- and \deltal-pinning. We also approach true critical current density by means of generalized inversion scheme, and the obtained result is in reasonable agreement with our model function. Proton irradiation effect on temperature dependence of current density and normalized relaxation rate can be summarized as doubling of barrier height at the beginning of flux creep.Comment: 6 pages, 4 figures. To be published in Phys. Rev.

Magnetic and transport properties of iron-platinum arsenide Ca10(Pt4-{\delta}As8)(Fe2-xPtxAs2)5 single crystal

We report superconducting properties of single crystalline Ca10(Pt4-{\delta}As8)(Fe2-xPtxAs2)5 by X-ray diffraction, magnetization, resistivity, and magneto-optical imaging measurements. The magnetization measurements reveal fish-tail hysteresis loop and relatively high critical current density Jc ~ 0.8\times105 A/cm2 at low temperatures. The exponential temperature dependence of Jc, which arises from nonlinear effective flux-creep activation energy, has been observed. Upper critical field determined by resistive transition shows a relatively large anisotropy. The magneto-optical images reveal homogenous current flow within the crystal.Comment: 6 pages, 6 figures, Accepted for publication in Phys. Rev.

Magnetic relaxation and collective vortex creep in FeTe0.6_{0.6}Se0.4_{0.4} single crystal

We study the vortex dynamics in high-quality FeTe$_{0.6}$Se$_{0.4}$ single crystal by performing magnetization measurements of the screening current density \emph{J}$_s$ and flux creep rate \emph{S}. Temperature dependence of \emph{S} shows a plateau in the intermediate temperature region with a high creep rate $\sim$ 0.03, which is interpreted in the framework of the collective creep theory. A crossover from elastic to plastic creep is observed. The glassy exponent and barrier height for flux creep are directly determined by extended Maley's method. \emph{J}$_s$ with flux creep, obtained from magnetic hysteresis loops, is successfully reproduced based on the collective creep analysis. We also approach critical current density without flux creep by means of the generalized inversion scheme, which proves that the $\delta$\emph{l} and $\delta$\emph{T}$_c$ pinning coexist in FeTe$_{0.6}$Se$_{0.4}$ single crystal.Comment: 6 pages, 5 figure