Satoshi Watauchi

Single crystals of Ce-doped (La,Pr)OBiS₂ superconductors, the multinary rare-earth elements substituted ROBiS₂, were successfully grown. The grown crystals typically had a size of 1–2 mm and a plate-like shape with a well-developed c-plane. The c-axis lattice constants of the obtained (La,Ce,Pr)OBiS₂ single crystals were approximately 13.6–13.7 Å, and the superconducting transition temperature was 1.23–2.18 K. Valence fluctuations of Ce and Pr were detected through X-ray absorption spectroscopy analysis. In contrast to (Ce,Pr)OBiS₂ and (La,Ce)OBiS₂, the superconducting transition temperature of (La,Ce,Pr)OBiS₂ increased with the increasing concentrations of the tetravalent state at the R-site.

The shape of the silicon crystal, whether spiral or cylindrical, was grown using the infrared convergent-heating floating-zone method, and the stability of the molten zone depended on the position of the mirror-lamp (M-L) system with respect to the grown crystal and molten zone. Through experiment, we studied the changes in the molten zone shape: the melt/feed, melt/gas, and melt/crystal interface shapes of the silicon molten zone under different positions of the M-L system. Although conventional parameters such as convexities (h/r) of the interface toward the melt were found to be independent of the position of the M-L system, the asymmetry of the zone length, L, was found to be inversely proportional to the distance of the M-L system from the center of the molten zone. A spiral crystal would be grown in this case. We introduce some parameters such as growth interface angle (δ), triple point angle (TPA), meniscus angle (MA), and altitude of the interface curvature (a_C) to characterize the axis symmetry of the melt/gas and melt/crystal interfaces. The variations in TPA, MA, and a_C were significantly reduced when the M-L system was shifted to a distant position from the center of the molten zone. On the other hand, the variations in δ were independent of the position of the M-L system. Thus, a symmetric molten zone was observed when the M-L system was at a distant rather than a close position. These behaviors were validated through observations of the molten zone stability and the crystal shape.

The effects of lamp power and mirror position on the feed–melt and crystal–melt interfaces during the growth of a silicon crystal were examined using the mirror-shifting-type infrared convergent-heating floating-zone (IR-FZ) method.

Superconductivity of Fe0.95Te0.85S0.15 was induced and enhanced by immersing the as-grown (non-superconducting) sample into sulfuric acid. Dissolved Fe was detected in the sulfuric acid treatment, indicating that excess Fe was removed from the Fe0.95Te0.85S0.15 sample