Atsushi Omachi

Correlation between crystal structure and superconductivity in LaO0.5F0.5BiS2

Abstract Correlation between crystal structure and superconducting properties of the BiS2-based superconductor LaO0.5F0.5BiS2 was investigated. We have prepared LaO0.5F0.5BiS2 polycrystalline samples with various lattice constants. It was found that high-pressure annealing generated uniaxial strain along the c axis. Further, the highly-strained sample showed higher superconducting properties. We concluded that the uniaxial strain along the c axis was positively linked with the enhancement of superconductivity in the LaO1−xFxBiS2 system.

Enhancement of Tc by Uniaxial Lattice Contraction in BiS2-Based Superconductor PrO0.5F0.5BiS2

We investigated the crystal structure and superconducting properties of As-grown and high-pressure-annealed PrO0.5F0.5BiS2. We found that the high-pressure annealing generates uniaxial lattice contraction along the c axis. Both As-grown and high-pressure-annealed PrO0.5F0.5BiS2 show bulk superconductivity. The Tc of PrO0.5F0.5BiS2 is clearly enhanced from Tczero = 3.6 K to Tczero = 5.5 K by high-pressure annealing. Unexpectedly, the semiconducting characteristics is relatively enhanced by high-pressure annealing. Namely, we assume that the enhancement of Tc can not be understood by an increase of electron carriers. Having considered these facts, we conclude that the enhancement of Tc correlates with uniaxial lattice contraction along the c axis in PrO0.5F0.5BiS2.

Enhancement of thermoelectric properties by Se substitution in layered bismuth-chalcogenide LaOBiS2-xSex

We have investigated the thermoelectric properties of the novel layered bismuth chalcogenides LaOBiS2-xSex. The partial substitution of S by Se produced the enhancement of electrical conductivity (metallic characteristics) in LaOBiS2-xSex. The power factor largely increased with increasing Se concentration. The highest power factor was 4.5 μW/cmK2 at around 470 °C for LaOBiS1.2Se0.8. The obtained dimensionless figure-of-merit (ZT) was 0.17 at around 470 °C in LaOBiS1.2Se0.8.

High-temperature thermoelectric properties of novel layered bismuth-sulfide LaO1−xFxBiS2

We have investigated the high-temperature thermoelectric properties of the layered compound LaO1−xFxBiS2. The electrical resistivity of LaOBiS2 showed an anomalous behavior; a metal-semiconductor transition was observed around 270 K. It was found that the value of the electrical resistivity decreased with F substitution. The Seebeck coefficient decreased with increasing F concentration. The highest power factor of 1.9 μW/cm K2 at 480 °C was obtained for LaOBiS2.

Increase in Tc and Change of Crystal Structure by High-Pressure Annealing in BiS2-Based Superconductor CeO0.3F0.7BiS2

Recently, several types of BiS2-based superconductor such as Bi4O4S3, REO1−xFxBiS2 (RE: rare earth) and Sr1−xLaxFBiS2 have been discovered. In this study, we have investigated the crystal structure and the superconducting properties for two kinds of polycrystalline samples (as-grown and high-pressure-annealed samples) of the BiS2-based superconductor CeO0.3F0.7BiS2. We found that both the as-grown and the high-pressure-annealed CeO0.3F0.7BiS2 samples show bulk superconductivity. A higher Tc was observed in the high-pressure-annealed sample. The Tc of CeO0.3F0.7BiS2 increased from Tczero=

Thermoelectric properties of new Bi-chalcogenide layered compounds

T_{\mathrm {c}}^{\text {zero}}=

Importance of uniaxial compression for the appearance of superconductivity in NdO1−xFxBiS2

2.7 K to Tczero=

Chemical pressure effect on Tc in BiS2-based Ce1−xNdxO0.5F0.5BiS2

T_{\mathrm {c}}^{\text {zero}}=

Element Substitution Effect on Superconductivity in BiS2-Based NdO1−xFxBiS2

3.7 K by high-pressure annealing. The lattice constant of a and c axes did not show remarkable differences between the as-grown and the high-pressure-annealed samples. Nevertheless, the peak symmetry of the (200) peak seemed to become more symmetric while the (004) peak did not show such a difference, indicating that the crystal structure within the ab-plane changed to a higher-symmetric phase, a perfect tetragonal, by high-pressure annealing in CeO0.3F0.7BiS2.

CONDENSED MATTER PHYSICS | REVIEW ARTICLE Thermoelectric properties of new Bi-chalcogenide layered compounds

Abstract The layered Bi-chalcogenide compounds have been drawing much attention as a new layered superconductor family since 2012. Due to the rich variation of crystal structure and constituent elements, the development of new physics and chemistry of the layered Bi-chalcogenide family and its applications as functional materials have been expected. Recently, it was revealed that the layered Bi chalcogenides can show a relatively high thermoelectric performance (ZT = 0.36 in LaOBiSSe at ~650 K). Here, we show the crystal structure variation of the Bi-chalcogenide family and their thermoelectric properties. Finally, the possible strategies for enhancing the thermoelectric performance are discussed on the basis of the experimental and the theoretical facts reviewed here.