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Novel BiS2-based superconductors LaO1−xFxBiS2 prepared by a high-pressure synthesis technique were systematically studied. It was found that the high-pressure annealing strongly shrank the lattice as compared to the LaO1−xFxBiS2 samples prepared by conventional solid-state reaction at ambient pressure. Bulk superconductivity was observed within a wide F concentration range of x = 0.2-0.7. On the basis of those results, we have established a phase diagram of LaO1−xFxBiS2.

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.

The critical temperature Tc and the critical current density Jc determine the limits to large-scale superconductor applications. Superconductivity emerges at Tc. The practical current-carrying capability, measured by Jc, is the ability of defects in superconductors to pin the magnetic vortices, and that may reduce Tc. Simultaneous increase of Tc and Jc in superconductors is desirable but very difficult to realize. Here we demonstrate a route to raise both Tc and Jc together in iron-based superconductors. By using low-energy proton irradiation, we create cascade defects in FeSe0.5Te0.5 films. Tc is enhanced due to the nanoscale compressive strain and proximity effect, whereas Jc is doubled under zero field at 4.2 K through strong vortex pinning by the cascade defects and surrounding nanoscale strain. At 12 K and above 15 T, one order of magnitude of Jc enhancement is achieved in both parallel and perpendicular magnetic fields to the film surface.