Keita Deguchi

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.

Superconductivity in Novel BiS2-Based Layered Superconductor LaO1-xFxBiS2

Layered superconductors have provided some interesting fields in condensed matter physics owing to the low dimensionality of their electronic states. For example, the high-Tc (high transition temperature) cuprates and the Fe-based superconductors possess a layered crystal structure composed of a stacking of spacer (blocking) layers and conduction (superconducting) layers, CuO2 planes or Fe-Anion layers. The spacer layers provide carriers to the conduction layers and induce exotic superconductivity. Recently, we have reported superconductivity in the novel BiS2-based layered compound Bi4O4S3. It was found that superconductivity of Bi4O4S3 originates from the BiS2 layers. The crystal structure is composed of a stacking of BiS2 superconducting layers and the spacer layers, which resembles those of high-Tc cuprate and the Fe-based superconductors. Here we report a discovery of a new type of BiS2-based layered superconductor LaO1-xFxBiS2, with a Tc as high as 10.6 K.

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.

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.

Pressure-Induced Enhancement of Superconductivity and Structural Transition in BiS2-Layered LaO1¹xFxBiS2

BiS2-based LaO1−xFxBiS2 (x = 0.5) becomes superconductive at Tc = 2.5 K. Electrical resistivity and magnetization measurements are performed under pressure to determine the pressure dependence of the superconducting transition temperature Tc up to 18 GPa. We observe that Tc abruptly increases from 2.5 to 10.7 K at a pressure of 0.7 GPa. According to high-pressure X-ray diffraction measurements of LaO1−xFxBiS2 (x = 0 and 0.5), a structural phase transition from tetragonal to monoclinic also occurs at ∼0.8 GPa. We consider that the pressure-induced enhancement of superconductivity for LaO1−xFxBiS2 is caused by the structural phase transition.

Structural Analysis and Superconducting Properties of F-Substituted NdOBiS2 Single Crystals

F-substituted NdOBiS2 superconducting single crystals were grown using CsCl/KCl flux. This is the first example of the single-crystal growth of a BiS2-based superconductor. The obtained single crystals had a plate-like shape with a size of 1–2 mm and a well-developed \(ab\)-plane. The crystal structure of the grown crystals was determined by single-crystal X-ray diffraction analysis to be the tetragonal space group \(P4/nmm\) (#129) with \(a=3.996(3)\) and \(c=13.464(6)\) A. The chemical formula of the grown crystals was approximately Nd0.98\pm 0.06O0.7\pm 0.1F0.3\pm 0.1Bi0.98\pm 0.04S2, and Cs, K, and Cl were not detected in the grown crystals by electron probe microanalysis. The grown crystals had a critical temperature of approximately 5 K. The superconducting anisotropy of the single crystals was estimated to be about 30 from the effective mass model and the upper critical field.

Physics and chemistry of layered chalcogenide superconductors

Abstract Structural and physical properties of layered chalcogenide superconductors are summarized. In particular, we review the remarkable properties of the Fe-chalcogenide superconductors, FeSe and FeTe-based materials. Furthermore, we introduce the recently discovered BiS2-based layered superconductors and discuss their prospects.

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.

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 1027 A/cm2 at 4.2 K. The superconducting transition temperature Tczero 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.

Moisture-induced superconductivity in FeTe 0.8 S 0.2

Moisture-induced superconductivity was observed in