Yasuna Kawasaki

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 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.

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.

Clarification as to why alcoholic beverages have the ability to induce superconductivity in Fe1+dTe1?xSx

To elucidate the mechanism as to why alcoholic beverages can induce superconductivity in Fe1+dTe1?xSx samples, we performed component analysis and found that a weak acid such as an organic acid has the ability to induce superconductivity. Inductively coupled plasma spectroscopy was performed on weak acid solutions post-annealing. We found that the mechanism of inducement of superconductivity in Fe1+dTe1?xSx is the deintercalation of excess Fe from the interlayer sites.

One-step synthesis of KxFe2−ySe2 single crystal for high critical current density

We have established a simple process that allows for the one-step synthesis of KxFe2-ySe2 single crystals, which exhibit high critical current density Jc. The post annealing and quenching technique has improved the homogeneity of as-grown crystals, resulting in full shielding of the external magnetic field. The quenched crystals show a superconducting transition at Tconset = 32.9 K and Tczero = 32.1 K. The upper critical fields \mu_{0}Hc2(0) for H//ab and H//c are estimated to be ~206 and ~50 T, respectively. The critical current densities Jc for H//ab and H//c reach as high as 1.0\times10^{5} and 3.4\times10^{4} A/cm2 at 5 K. Furthermore, Jc exhibits a high field performance and a significantly weak temperature dependence up to 5 T, suggesting strong pinning. These results demonstrate that KxFe2-ySe2 would be a promising candidate material for practical applications.

Electrochemical Synthesis of Iron-Based Superconductor FeSe Films

Superconducting FeSe films were successfully fabricated using electrochemical synthesis methods. The composition ratio of Fe and Se was controlled by electric potential and pH value. We found that the FeSe film deposited at an electric potential of -1.75 V vs Ag/AgCl at a pH value of 2.3 exhibits a superconducting transition at 3.5 K. The electrochemical synthesis technique that we have developed will offer many advantages for applications.

Enhancement of superconducting properties in FeSe wires using a quenching technique

Enhancements of superconducting properties were observed in FeSe wires using a quenching technique. Zero resistivity was achieved at about 10 K in quenched wires, which is about 2 K higher than that of polycrystalline FeSe bulk. Furthermore, transport Jc of quenched wires showed three times higher than that of furnace-cooled wires. In contrast, the quenched polycrystalline FeSe bulks did not show the enhancement of Tc. The quenching technique is greatly promising for fabricating FeSe wires with high Tc and high Jc, and quenched FeSe wires have high potential for superconducting wire applications.

Pressure Study of the New Iron-Based Superconductor K0.8Fe2Se2

We investigated pressure effects on transition temperature (Tc) of the new iron-based superconductor K0.8Fe2Se2 using a BeCu/NiCrAl hybrid-type clamped piston-cylinder cell. The Tc(onset) was 33K at 0.85 GPa. With increasing pressure, Tc(onset) gradually increased and reached 36.6 K at 2.03 GPa.

Evidence of Inhomogeneous Superconductivity in FeTe₁₋xSe[x] by Scotch-Tape Method

We have fabricated thin films of FeTe 1- x Se x using a scotch-tape method. The superconductivities of the thin films are different from each other although these films were fabricated from the same bulk sample. The result clearly presents the inhomogeneous superconductivity in FeTe 1- x Se x . The difference might arise from inhomogeneity due to the excess Fe concentration. The resistivity of a thin film with low excess Fe shows good superconductivity with the sharp superconducting-transition width and more isotropic superconductivity.

Comprehensive studies for the crystal structures and electronic properties of the superconducting system Fe1 + δSe1 − xTex with \delta \simeq 0.037 and x \simeq 0.55

Structural aspects and electronic properties for the Fe1 + δSe1 − xTex system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition and , are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for 125Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin–lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.Structural aspects and electronic properties for the Fe(1 + δ)Se(1 - x)Te(x) system with 0.04 ≤ δ ≤ 0.08 and 0.5 ≤ x ≤ 1, especially with the superconducting composition δ is approximately equal to 0.037 and x is approximately equal to 0.55, are investigated comprehensively with x-ray four-circle diffraction and through measurements of electrical resistivity, thermoelectric power, magnetic susceptibility and nuclear magnetic resonance for (125)Te. The crystal structures with an excess Fe site are refined precisely with obvious constraints. For the superconducting composition, the transport properties are explained in terms of the two-band model, where an electron carrier band gives a linear-in-T resistivity and another hole band leads to nearly temperature-independent behaviour. The magnetic susceptibility and the Knight shift are explained with the idea that the electron correlation is enhanced with increasing x and it is reduced with annealing. The spin-lattice relaxation rates for the normal state that show the apparent Korringa relation may also be understood in this framework. These evidences suggest that the superconductivity may emerge in a regime where the correlation is relatively weak in this system.