Osuke Miura

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.

Critical Current Density Enhancement around a Matching Field in ErBa2Cu3O7-δ Films with BaZrO3 Nano-Rods

Field angular dependences of critical current density are measured for as-grown high crystalline quality ErBa2Cu3O7-δ films with BaZrO3 nano-rods. Large critical current density enhancement is observed. The magnetic field where the enhancement is observed coincides with the matching field calculated from the density of BaZrO3 nano-rods. The largest enhancement is observed in B ∥c-axis of ErBa2Cu3O7-δ films. This anisotropic critical current density enhancement is thought to be caused by the direction of BaZrO3 nano-rods which grow along the c-axis of the films. BaZrO3 nano-rods in the films will promise the application of coated conductors in a magnetic field.

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.

Transport properties of single- and three-core FeSe wires fabricated by a novel chemical-transformation PIT process

We fabricated single- and three-core superconducting FeSe wires by a novel process based on a chemical transformation from hexagonal FeSe1 + d (non-superconducting) to tetragonal FeSe (superconducting) via an optimal supply of Fe from the Fe sheath by annealing. This process enhanced the packing density of the core inside the sheath, owing to an expansion of lattice volume via a chemical transformation from high-density hexagonal FeSe1 + d to low-density tetragonal FeSe. The obtained wire showed superconductivity below ~ 10 K and the obtained transport critical current densities at 0 T and 4.2 K were 588 A cm − 2 for the three-core wire and 218 A cm − 2 for the single-core wire.

Crystal Structure, Lattice Vibrations,and Superconductivity of LaO1-xFxBiS2

Neutron scattering measurements have been performed on polycrystalline samples of the newly discovered layered superconductor LaO0:5F0:5BiS2, and its nonsuperconducting parent compound LaOBiS2. The crystal structures and vibrational modes have been examined. Upon F-doping, while the lattice contracts signicantly along c and expands slightly along a, the buckling of the BiS2 plane remains almost the same. In the inelastic measurements, a large dierence in the high energy phonon modes was observed upon F substitution. Alternatively, the low energy modes remain almost unchanged between non-superconducting and superconducting states either by F- doping or by cooling through the transition temperature. Using density functional perturbation theory we identify the phonon modes, and estimate the phonon density of states. We compare these calculations to the current measurements and other theoretical studies of this new superconducting material.

In-plane chemical pressure essential for superconductivity in BiCh2-based (Ch: S, Se) layered structure.

BiCh2-based compounds (Ch: S, Se) are a new series of layered superconductors, and the mechanisms for the emergence of superconductivity in these materials have not yet been elucidated. In this study, we investigate the relationship between crystal structure and superconducting properties of the BiCh2-based superconductor family, specifically, optimally doped Ce1−xNdxO0.5F0.5BiS2 and LaO0.5F0.5Bi(S1−ySey)2. We use powder synchrotron X-ray diffraction to determine the crystal structures. We show that the structure parameter essential for the emergence of bulk superconductivity in both systems is the in-plane chemical pressure, rather than Bi-Ch bond lengths or in-plane Ch-Bi-Ch bond angle. Furthermore, we show that the superconducting transition temperature for all REO0.5F0.5BiCh2 superconductors can be determined from the in-plane chemical pressure.

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.

Bulk Superconductivity in Bi4O4S3 Revealed by Specific Heat Measurement

Specific heat experiments on a well-characterized polycrystalline sample of the BiS2 based superconductor Bi4O4S3 revealed that it shows a crear specific heat anomaly at about Tc = 4.4 K, consistent with Tc from the resistivity and dc susceptibility. This observation indicates the superconductivity of Bi4O4S3 to be bulk in nature.

In-plane Charge Fluctuations in Bismuth Sulfide Superconductors

The local atomic structure of the new nonmagnetic superconducting system