Takano Yoshihiko

New Member of BiS2-Based Superconductor NdO1-xFxBiS2

We have successfully synthesized the new BiS2-based superconductor NdOBiS2 by F doping. This compound is composed of superconducting BiS2 layers and blocking NdO layers, which indicates that the BiS2 layer is analogous to the CuO2 layer in cuprates or to the Fe–As layer in Fe-based superconductors. We can obtain NdO1-xFxBiS2 with bulk superconductivity by a solid-state reaction. Therefore, NdO1-xFxBiS2 should be a suitable material for elucidating the mechanism of superconductivity in the BiS2 layer.

Pressure Study of BiS2-Based Superconductors Bi4O4S3 and La(O,F)BiS2

We report the electrical resistivity measurements under pressure for the recently discovered BiS2-based layered superconductors Bi4O4S3 and La(O,F)BiS2. In Bi4O4S3, the transition temperature Tc decreases monotonically without a distinct change in the metallic behavior in the normal state. In La(O,F)BiS2, on the other hand, Tc initially increases with increasing pressure and then decreases above ? 1 GPa. The semiconducting behavior in the normal state is suppressed markedly and monotonically, whereas the evolution of Tc is nonlinear. The strong suppression of the semiconducting behavior without doping in La(O,F)BiS2 suggests that the Fermi surface is located in the vicinity of some instability. In the present study, we elucidate that the superconductivity in the BiS2 layer favors the Fermi surface at the boundary between the semiconducting and metallic behaviors.We report the electrical resistivity measurements under pressure for the recently discovered BiS 2 -based layered superconductors Bi 4 O 4 S 3 and La(O,F)BiS 2 . In Bi 4 O 4 S 3 , the transition te...We report the electrical resistivity measurements under pressure for the recently discovered BiS 2 -based layered superconductors Bi 4 O 4 S 3 and La(O,F)BiS 2 . In Bi 4 O 4 S 3 , the transition temperature T c decreases monotonically without a distinct change in the metallic behavior in the normal state. In La(O,F)BiS 2 , on the other hand, T c initially increases with increasing pressure and then decreases above ∼1 GPa. The semiconducting behavior in the normal state is suppressed markedly and monotonically, whereas the evolution of T c is nonlinear. The strong suppression of the semiconducting behavior without doping in La(O,F)BiS 2 suggests that the Fermi surface is located in the vicinity of some instability. In the present study, we elucidate that the superconductivity in the BiS 2 layer favors the Fermi surface at the boundary between the semiconducting and metallic behaviors.

Coexistence of Bulk Superconductivity and Magnetism in CeO1−xFxBiS2

We show the observation of the coexistence of bulk superconductivity and ferromagnetism in CeO1-xFxBiS2(x = 0 - 1.0) prepared by annealing under high-pressure. In CeO1-xFxBiS2 system, both superconductivity and two types of ferromagnetism with respective magnetic transition temperatures of 4.5 K and 7.5 K are induced upon systematic F substitution. This fact suggests that carriers generated by the substitution of O by F are supplied to not only the BiS2 superconducting layers but also the CeO blocking layers. Furthermore, the highest superconducting transition temperature is observed when the ferromagnetism is also enhanced, which implies that superconductivity and ferromagnetism are linked to each other in the CeO1-xFxBiS2 system.

Successive Phase Transitions under High Pressure in FeTe0.92

We performed magnetization and electrical resistivity measurements under high pressures of up to 19 GPa for FeTe0.92. The compound shows an anomaly in magnetization and resistivity at atmospheric pressure due to a structural distortion accompanied by a magnetic transition. We also observed magnetic and resistive anomalies under high pressure, suggesting that two pressure-induced phases exist at a low temperature. Unlike in FeAs-based compounds, no superconductivity was detected under high pressures of up to 19 GPa, although the anomaly at atmospheric pressure was suppressed by applying pressure.

Pressure-Induced Superconductivity in BiS2-Based EuFBiS2

We measured the electrical resistivity of a BiS2-based compound EuFBiS2 under high pressure. Polycrystalline EuFBiS2 shows insulator-metal transition and pressure-induced superconductivity above 0.7 GPa. The superconducting transition temperature increases with increasing applied pressure and shows the maximum value around 8.6 K at 1.8 GPa.We measured the electrical resistivity of the BiS2-based compound EuFBiS2 under high pressure. Polycrystalline EuFBiS2 shows insulator–metal transition and pressure-induced superconductivity above 0.7 GPa. The superconducting transition temperature increases with increasing applied pressure and shows a maximum value around 8.6 K at 1.8 GPa.

Effect of the Indium Addition on the Superconducting Property and the Impurity Phase in Polycrystalline SmFeAsO1-xFx

We report enhancement in the magnetic critical current density of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic Jc is around 25 kA/cm2 at 4.2 K under self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of the superconducting grains and a little of amorphous FeAs compounds. These areas randomly co-exist and amorphous areas are located between superconducting grains. Therefore, the superconducting current is prevented by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the bringing together the superconducting grains. It means the total contact surfaces of grains are increased. We suggest that the enhancement of the magnetic critical current density is a direct effect of the indium addition.We report the increase in the magnetic critical current density (\(J_{\text{c}}\)) of indium added polycrystalline SmFeAsO1-xFx. The value of magnetic \(J_{\text{c}}\) is around \(2.5 \times 10^{4}\) A/cm2 at 4.2 K under a self-magnetic field. Polycrystalline SmFeAsO1-xFx is mainly composed of superconducting grains and a little amorphous FeAs compounds. These components randomly coexist and amorphous areas are located between superconducting grains. Therefore, superconducting current is prevented from flowing by the amorphous areas. In this study, it is found that indium addition to polycrystalline SmFeAsO1-xFx removes these amorphous areas and induces the clustering of the superconducting grains. This means that the total contact surface area of grains increases. We suggest that the increase in the magnetic \(J_{\text{c}}\) is a direct effect of the indium addition.

α-FeAs-Free SmFeAsO1-xFx by Low Temperature Sintering with Slow Cooling

We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 h and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96%. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950, 900, 850, 800, 750, and 700 °C. It is found that fluorine is substituted not only at the maximum heating temperature but also during the cooling process. The low temperature sintering with slow cooling is very effective to obtain a homogeneous SmFeAsO1-xFx with high fluorine concentration.We obtained amorphous-FeAs-free SmFeAsO1-xFx using a low temperature sintering with slow cooling. SmFeAsO1-xFx is sintered at 980 °C for 40 h and cooled slowly down to 600 °C. The low temperature sintering suppresses the formation of amorphous FeAs, and the slow cooling introduces much fluorine into SmFeAsO1-xFx. The superconductivity of this sample appears at 57.8 K and the superconducting volume fraction reaches 96%. To study the change of fluorine concentration during the cooling process, samples are quenched by water at 950, 900, 850, 800, 750, and 700 °C. It is found that fluorine is substituted not only at the maximum heating temperature but also during the cooling process. The low temperature sintering with slow cooling is very effective to obtain a homogeneous SmFeAsO1-xFx with high fluorine concentration.

Phase-Separation Control of KxFe2-ySe2 Superconductor through Rapid-Quenching Process

K

Transport Properties of Hydrogen-Terminated Silicon Surface Controlled by Ionic-Liquid Gating

_x

Single Crystalline MgB2 Superconductor

Fe