Flux avalanches in Nb superconducting shifted strip arrays

Flux penetrations into three-dimensional Nb superconducting strip arrays, where two layers of strip arrays are stacked by shifting a half period, are studied using a magneto-optical imaging method. Flux avalanches are observed when the overlap between the top and bottom layers is large even if the width of each strip is well below the threshold value. In addition, anomalous linear avalanches perpendicular to the strip are observed in the shifted strip array when the overlap is very large and the thickness of the superconductor is greater than the penetration depth. We discuss possible origins for the flux avalanches, including linear ones, by considering flux penetration calculated by the Campbell method assuming the Bean model.

Annealing, acid, and alcoholic beverage effects on Fe1+yTe0.6Se0.4

We have systematically investigated and compared different methods to induce superconductivity in iron chalcogenide Fe1+yTe0.6Se0.4 including annealing in vacuum, N2, O2, I2 atmosphere, and immersing samples into acid and alcoholic beverages. Vacuum and N2 annealing are proved to be ineffective to induce superconductivity in Fe1+yTe0.6Se0.4 single crystal. O2 and I2 annealing, acid and alcoholic beverages can induce superconductivity by oxidizing the excess Fe in the sample. Superconductivity in O2 annealed sample is in bulk nature, while I2, acid and alcoholic beverages can only induce superconductivity near the surface. By comparing different effects of O2, I2, acid and alcoholic beverages, we propose a scenario to explain how the superconductivity is induced in the non-superconducting as-grown Fe1+yTe0.6Se0.4.

Suppression of critical temperature in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ with point defects introduced by proton irradiation

We report the effect of 3 MeV proton irradiation on the suppression of the critical temperature $T_{c}$ in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals at under-, optimal-, and over-doping levels. We find that $T_{c}$ decreases and residual resistivity increases monotonically with increasing dose. We also find no upturn in low-temperature resistivity in contrast with the $yen alpha$-particle irradiated NdFeAs(O,F), which suggests that defects induced by the proton irradiation behave as nonmagnetic scattering centers. The critical scattering rate for all samples estimated by three different ways is much higher than that expected in $s_{yen pm}$-pairing scenario based on inter-band scattering due to antiferro-magnetic spin fluctuation.

Superconductivity at T_c ~ 14 K in Single Crystalline FeTe$_{0.61}$Se$_{0.39}$

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.

Enhancement of Critical Current Densities in Co-Doped BaFe$_{2}$As$_{2}$ with Columnar Defects Introduced by Heavy-Ion Irradiation

We report the first realization of columnar defects in Co-doped BaFe$_{2}$As$_{2}$ single crystals by heavy-iron 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 BaFe$_{2}$As$_{2}$ and cuprate superconductors.