Shunsuke Tsuda

Superconductivity at 27K in tetragonal FeSe under high pressure

A huge enhancement of the superconducting transition temperature Tc was observed in a tetragonal FeSe superconductor under high pressure. The onset temperature was as high as 27K at 1.48GPa and the pressure coefficient showed an extremely high value of 9.1K∕GPa. The upper critical field Hc2 was estimated to be ∼72T at 1.48GPa. Because of the high Hc2, the FeSe system can be applied to superconducting wire rods.

Anion height dependence of Tc for the Fe-based superconductor

We have established a plot of the anion height dependence of superconducting transition temperature Tc for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38??. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and searching for the new Fe-based superconductors with higher? Tc.

Substitution Effects on FeSe Superconductor

We have investigated the effect of atomic substitutions in the FeSe system, which exhibits the simplest crystal structure among the iron-based superconductors. An enhancement of the superconducting transition temperature T c was observed with the substitution of S or Te for Se; the T c increased with S substitution by up to 20%, and also increased with Te substitution up to 75%. In contrast, Co or Ni substitutions for the Fe site significantly suppressed superconductivity. In this work we present a detailed description of the substitution technique employed to determine T c in the FeSe system.

Superconductivity in S-substituted FeTe

We synthesized a superconducting system of FeTe1−xSx with a PbO-type structure. It has the simplest crystal structure among iron-based superconductors. The superconducting transition temperature is about 10 K when x is 0.2. The upper critical field Bc2(0) was estimated to be ~70 T. The coherent length was calculated to be ~2.2 nm. Since FeTe1−xSx is composed of nontoxic elements, this material is a candidate for applications that will further advance research on iron-based superconductors.

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.

Fabrication of the Iron-Based Superconducting Wire Using Fe(Se,Te)

We have fabricated the Fe(Se,Te) superconducting wire by a special process based on a powder-in-tube method. The pure Fe tube plays the role of not only the sheath but also the raw material for synthesizing the superconducting phases. We succeeded in observing zero resistivity current on the current?voltage measurements for the Fe(Se,Te) wire. Introduction of the pinning centers and fabricating a multi-core wire will enhance the critical current density for the next step.

Evolution of superconductivity by oxygen annealing in FeTe0.8S0.2

Oxygen annealing dramatically improved the superconducting properties of solid-state–reacted FeTe0.8S0.2, which showed only a broad onset of superconducting transition just after the synthesis. The zero resistivity appeared and the transition temperature Tczero reached 8.5 K by the oxygen annealing at 200 °C. The shielding volume fraction was also enhanced from 0 to almost 100%. The lattice constants were compressed by the oxygen annealing, indicating that the evolution of bulk superconductivity in FeTe0.8S0.2 was correlated to the shrinkage of the lattice.

Moisture-induced superconductivity in FeTe 0.8 S 0.2

Moisture-induced superconductivity was observed in

Superconductivity in oxygen-annealed FeTe1−xSx single crystal

{\text{FeTe}}_{0.8}{\text{S}}_{0.2}

Resistivity reduction of boron-doped multiwalled carbon nanotubes synthesized from a methanol solution containing boric acid

. With exposing the sample to the air, the zero-resistivity temperature and the superconducting volume fraction were enhanced up to 7.2 K and 48.5%, respectively, while the as-grown sample showed only filamentary superconductivity. We concluded that the causes of the evolution of superconductivity were water-related ions and/or molecules because only the sample kept in water at room temperature for several days showed superconductivity. The speed of evolution of superconductivity was strongly enhanced by immersing the sample into the hot water.