Mizuguchi Yoshikazu

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.

Stabilization of High-Tc Phase of BiS2-Based Superconductor LaO0.5F0.5BiS2 Using High-Pressure Synthesis

High-quality polycrystalline samples of LaO0.5F0.5BiS2 were obtained using high-pressure synthesis technique. The LaO0.5F0.5BiS2 sample prepared by heating at 700 °C under 2 GPa showed superconductivity with superconducting transition temperatures (Tc) of \(T_{\text{c}}^{\text{onset}} = 11.1\) and \(T_{\text{c}}^{\text{zero}} = 8.5\) K in the electrical resistivity measurements and \(T_{\text{c}}^{\text{onset}} = 11.5\) and \(T_{\text{c}}^{\text{irr}} = 9.4\) K in the magnetic susceptibility measurements, which are obviously higher than those of the LaO0.5F0.5BiS2 polycrystalline samples obtained using conventional solid-state reaction. It was found that the high-Tc phase can be stabilized under high pressure and relatively-low annealing temperature. X-ray diffraction analysis revealed that the high-Tc phase possessed a small ratio of lattice constants of a and c, c/a.

Evolution of Superconductivity in BiS2-Based Superconductor LaO0.5F0.5Bi(S1−xSex)2

We have systematically investigated the crystal structure, magnetic susceptibility, and electrical resistivity of the BiS2-based superconductor LaO0.5F0.5Bi(S1−xSex)2 (x = 0–0.7). With expanding la...

Chemical Pressure Effect on Superconductivity of BiS2-Based Ce1−xNdxO1−yFyBiS2 and Nd1−zSmzO1−yFyBiS2

We have studied crystal structure and physical properties of REO1−yFyBiS2 (Ce1−xNdxO1−yFyBiS2 and Nd1−zSmzO1−yFyBiS2) with three different F concentration (y = 0.7, 0.5, and 0.3) to investigate relationship between the emergence of superconductivity and crystal structure in the REO1−yFyBiS2 series. REO1−yFyBiS2 is suitable for discussing chemical pressure effect on physical properties because the RE site at the blocking layer can be fully or partially substituted by various RE3+ ions, which could systematically tune the lattice volume. With increasing chemical pressure (with decreasing lattice volume), lattice constant of a-axis systematically decreases while lattice constant of c-axis does not show a remarkable change, indicating that the RE site substitution basically affect the lattice constant of a-axis. On the other hand, lattice constant of c-axis can be tuned by F concentration. On the basis of magnetic susceptibility measurements, we have obtained three kinds of superconductivity phase diagram wit...

SnAs-Based Layered Superconductor NaSn2As2

Superconducting behavior with exotic characteristics is often observed in materials with a layered two-dimensional crystal structure. Low dimensionality affects the electronic structure of these materials, potentially leading to high transition temperatures (Tc) and/or unconventional pairing mechanisms. In this letter, we report on superconductivity in layered tin arsenide NaSn2As2. The crystal structure consists of Sn2As2 bilayers, bound by the van der Waals forces and separated by Na+ ions. Measurements of electrical resistivity and specific heat confirm the bulk nature of superconductivity of NaSn2As2, with Tc of 1.3 K. Our results suggest that layered SnAs is a basic structure, offering another universality class in the family of layered superconductors. The results provide a new platform for the studies of physics and chemistry of low-dimensional superconductors with lone pair electrons.

Bulk Superconductivity Induced by In-Plane Chemical Pressure Effect in Eu0.5La0.5FBiS2−xSex

We have investigated the Se substitution effect on the superconductivity of optimally doped BiS2-based superconductor Eu0.5La0.5FBiS2. Eu0.5La0.5FBiS2−xSex samples with x = 0–1 were synthesized. With increasing x, in-plane chemical pressure is enhanced. For x ≥ 0.6, superconducting transitions with a large shielding volume fraction are observed in magnetic susceptibility measurements, and the highest Tc is 3.8 K for x = 0.8. From low-temperature electrical resistivity measurements, a zero-resistivity state is observed for all the samples, and the highest Tc is observed for x = 0.8. With increasing Se concentration, the characteristic electrical resistivity changes from semiconducting-like to metallic, suggesting that the emergence of bulk superconductivity is linked with the enhanced metallicity. A superconductivity phase diagram of the Eu0.5La0.5FBiS2−xSex superconductor is established.

Intrinsic Phase Diagram of Superconductivity in the BiCh2-Based System Without In-Plane Disorder

We have investigated the crystal structure and physical properties of LaO1−xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1−xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2–0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.We have investigated the crystal structure and physical properties of LaO1-xFxBiSSe to reveal the intrinsic superconductivity phase diagram of the BiCh2-based layered compound family. From synchrotron X-ray diffraction and Rietveld refinements with anisotropic displacement parameters, we clearly found that the in-plane disorder in the BiSSe layer was fully suppressed for all x. In LaO1-xFxBiSSe, metallic conductivity and superconductivity are suddenly induced by electron doping even at x = 0.05 and 0.1 with a monoclinic structure. In addition, x (F concentration) dependence of the transition temperature (Tc) for x = 0.2-0.5 with a tetragonal structure shows an anomalously flat phase diagram. With these experimental facts, we have proposed the intrinsic phase diagram of the ideal BiCh2-based superconductors with less in-plane disorder.