Satoshi Demura

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.

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.

Structural Analysis and Superconducting Properties of F-Substituted NdOBiS2 Single Crystals

F-substituted NdOBiS2 superconducting single crystals were grown using CsCl/KCl flux. This is the first example of the single-crystal growth of a BiS2-based superconductor. The obtained single crystals had a plate-like shape with a size of 1–2 mm and a well-developed \(ab\)-plane. The crystal structure of the grown crystals was determined by single-crystal X-ray diffraction analysis to be the tetragonal space group \(P4/nmm\) (#129) with \(a=3.996(3)\) and \(c=13.464(6)\) A. The chemical formula of the grown crystals was approximately Nd0.98\pm 0.06O0.7\pm 0.1F0.3\pm 0.1Bi0.98\pm 0.04S2, and Cs, K, and Cl were not detected in the grown crystals by electron probe microanalysis. The grown crystals had a critical temperature of approximately 5 K. The superconducting anisotropy of the single crystals was estimated to be about 30 from the effective mass model and the upper critical field.

Phase diagram and superconductivity at 58.1 K in α-FeAs-free SmFeAsO1−xFx

The phase diagram of SmFeAsO1−xFx in terms of x is exhibited in this study. Specimens of SmFeAsO1−xFx from x = 0 to x = 0.3 were prepared by low-temperature sintering with slow cooling. The low-temperature sintering suppresses the formation of the amorphous FeAs, which is inevitably produced as an impurity when using high-temperature sintering. Moreover, slow cooling is effective in obtaining a high fluorine concentration. The compositional change from feedstock composition is quite small after this synthesis. We can reproducibly observe a record superconducting transition for an iron-based superconductor at 58.1 K. This achievement of a high superconducting transition is due to the success in substituting a large amount of fluorine. A shrinking of the a lattice parameter caused by fluorine substitution is observed and the substitutional rate of fluorine changes at x = 0.16.

Role of the Ce valence in the coexistence of superconductivity and ferromagnetism of CeO1x FxBiS2 revealed by Ce L3 -edge x-ray absorption spectroscopy

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

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

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Determination of local atomic displacements in CeO1?xFxBiS2 system

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“Checkerboard Stripe” Electronic State on Cleaved Surface of NdO0.7F0.3BiS2 Probed by Scanning Tunneling Microscopy

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