Toshinori Ozaki

Evolution of superconductivity in LaO1−xFxBiS2 prepared by high-pressure technique

Novel BiS2-based superconductors LaO1-xFxBiS2 prepared by the high pressure synthesis technique were systematically studied. It was found that the high pressure annealing strongly 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.

Transport properties of the new Fe-based superconductor KxFe2Se2 (Tc=33 K)

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 tempera-ture were estimated to be 33 and 31.8 K, respectively. A high upper critical field of 192 T was obtained. The anisotropy of superconductivity of K0.8Fe2Se2 was ∼3.6. Both the high upper critical field and comparably low anisotropy are advantageous for applications under a high magnetic field.

Transport properties and microstructure of mono- and seven-core wires of FeSe1 − xTex superconductor produced by the Fe-diffusion powder-in-tube method

We report the successful fabrication of mono-xa0and seven-core superconducting wires of FeSe1 − xTex using an inxa0situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The reacted layers in these wires were found to have composite structure with the composition nearly FeSe and FeTe for the inner and outer layers, although a single layer of composition FeSe0.5Te0.5 was supposed to be formed. The self-field transport Jc values at 4.2xa0K were found to be 226.2xa0Axa0cm − 2 and 100.3xa0Axa0cm − 2 respectively for mono-xa0and seven-core wires. The Jcs of these wires dropped rapidly at low fields and then showed a gradual decrease with increasing magnetic fields. In addition, the seven-core wire showed higher Jc than the mono-core wire under higher magnetic fields, indicating that the seven-core wire of FeSe1 − xTex superconductor produced using the Fe-diffusion PIT method is advantageous for superconducting wire applications under high magnetic fields.

Alcoholic beverages induce superconductivity in FeTe1 − xSx

We found that hot alcoholic beverages were effective in inducing superconductivity in FeTe0.8S0.2. Heating the FeTe0.8S0.2 compound in various alcoholic beverages enhances the superconducting properties compared to a pure water–ethanol mixture as a control. Heating with red wine for 24xa0h leads to the largest shielding volume fraction of 62.4% and the highest zero resistivity temperature of 7.8xa0K. Some components present in alcoholic beverages, other than water and ethanol, have the ability to induce superconductivity in the FeTe0.8S0.2 compound.

Fabrication of binary FeSe superconducting wires by diffusion process

We report successful fabrication of multi- and mono-core FeSe wires with high transport critical current density Jc using a simple in-situ Fe-diffusion process based on the powder-in-tube (Fe-diffusion PIT) method. The seven-core wire showed transport Jc of as high as 1027u2009A/cm2 at 4.2u2009K. The superconducting transition temperature Tczero was observed at 10.5u2009K in the wire-samples, which is about 2u2009K 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.

Flux pinning properties and microstructure of SmBa2Cu3Oy thin films with systematically controlled BaZrO3 nanorods

We report a way of tuning the flux pinning properties by controlling the size and number density of BaZrO3 (BZO) nanorods without much degradation of the superconducting properties. The BZO nanorods in REBa2Cu3Oy superconducting films are known as promising c-axis-correlated pinning centers. We fabricated SmBa2Cu3Oy (SmBCO) films with BZO nanorods by a low-temperature growth technique (LTG-SmBCO+BZO films). With decreasing substrate temperature of the upper layer Tsupper in LTG-SmBCO+BZO films, the diameter of BZO nanorods decreased and their number density increased, leading to a high matching field Bϕ. Also, the considerable upturn shifts in the irreversibility field line and plateau regions in the magnetic field dependence of critical current density Jc were observed in the range from Bϕ/3 to Bϕ. These results indicate that a Bose-glass-like state of vortices localized on BZO nanorods emerges, after overcoming the vortex glass state of vortices, which are frozen on inherent pointlike disorders within t...

Transport properties of single- and three-core FeSe wires fabricated by a novel chemical-transformation PIT process

We fabricated single- and three-core superconducting FeSe wires by a novel process based on a chemical transformation from hexagonal FeSe1 + d (non-superconducting) to tetragonal FeSe (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced the packing density of the core inside the sheath, owing to an expansion of lattice volume via a chemical transformation from high-density hexagonal FeSe1 + d to low-density tetragonal FeSe. The obtained wire showed superconductivity below ~ 10 K and the obtained transport critical current densities at 0 T and 4.2 K were 588 A cm − 2 for the three-core wire and 218 A cm − 2 for the single-core wire.

Phase diagram and superconductivity at 58.1 K in α-FeAs-free SmFeAsO1−xFx

The phase diagram of SmFeAsO1−xFx in terms of x is exhibited in this study. Specimens of SmFeAsO1−xFx from x = 0 to x = 0.3 were prepared by low-temperature sintering with slow cooling. The low-temperature sintering suppresses the formation of the amorphous FeAs, which is inevitably produced as an impurity when using high-temperature sintering. Moreover, slow cooling is effective in obtaining a high fluorine concentration. The compositional change from feedstock composition is quite small after this synthesis. We can reproducibly observe a record superconducting transition for an iron-based superconductor at 58.1 K. This achievement of a high superconducting transition is due to the success in substituting a large amount of fluorine. A shrinking of the a lattice parameter caused by fluorine substitution is observed and the substitutional rate of fluorine changes at x = 0.16.

Effective Ex-situ Fabrication of F-Doped SmFeAsO Wire for High Transport Critical Current Density

We demonstrate the fabrication of superconducting SmFeAsO1-xFx (Sm-1111) wires by using the ex-situ powder-in-tube technique. Sm-1111 powder and a binder composed of SmF3, samarium arsenide, and iron arsenide were used to synthesize the superconducting core. Although the F content of Sm-1111 is reduced in the process of ex-situ fabrication, the binder compensates by sufficiently supplementing the F content, thereby preventing a decrease in the superconducting transition temperature and a shrinkage of the superconducting volume fraction. Thus, in the superconducting Sm-1111 wire with the binder, the transport critical current density reaches the highest value of ~4 kA/cm2 at 4.2 K.

Superconductivity in oxygen-annealed FeTe1−xSx single crystal

We investigated the S-doping-driven phase transition from antiferromagnetic to superconducting in FeTe1-xSx single crystals. The partial substitution of Te by S suppresses antiferromagnetism in Fe-square lattice. Superconductivity is induced by oxygen annealing for only FeTe1-xSx in which the long-range magnetic ordering is suppressed. To realize superconductivity in FeTe1-xSx, both S concentration enough to suppress antiferromagnetism and oxygen annealing are required. Anisotropy of superconductivity in oxygen-annealed FeTe0.886S0.114 was estimated to be 1.17.