We synthesized the new Fe-based superconductor K0.8Fe2Se2 single crystals. The obtained single crystal exhibited a sharp superconducting transition, and the onset and zero-resistivity temperature were estimated to be 33 and 31.8 K, respectively. A high upper critical field of 192 T was obtained. Anisotropy of superconductivity of K0.8Fe2Se2 was ~3.6. Both the high upper critical field and comparably low anisotropy are advantageous for the applications under high magnetic field.

Phase diagrams of as-grown and O 2 -annealed FeTe 1-x Se x determined from magnetic susceptibility measurement were obtained. For as-grown samples, the antiferromagnetic order was fully suppressed in the range region x ≥ 0.15, and weak superconductivity appeared when x ≥ 0.1.Beginning at x = 0.5, weak superconductivity was found to evolve into bulk superconductivity.Interestingly, for O 2 -annealed samples, complete suppression of magnetic order and the occurrence of bulk superconductivity were observed when x ≥ 0.1. We found that O 2 -annealing induces bulk superconductivity for FeTe 1-x Se x . Oxygen probably plays a key role in the suppression of the magnetic order and the appearance of bulk superconductivity.

(Abstract)We report successful fabrication of multi-and mono-core FeSe wires with high transport critical current density J c using a simple in-situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The seven-core wire showed transport J c of as high as 1027 A/cm 2 at 4.2 K. The superconducting transition temperature T c zero was observed at 10.5 K in the wire-samples, which is about 2 K higher than that of bulk FeSe. The Fe-diffusion PIT method is suitable for fabricating multi-core wires of the binary FeSe superconductors with superior properties.3