Improvement of Intragrain Critical Current Density in High-TemperatureSuperconductor by Self-Assembled Two-Dimensional Nanoplane Pinning Centers

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Sr2+ sites by an Al2O3-seeded glassy quenched platelet method. We found that Bi-based superconducting whiskers whose Sr2+ sites of 25% were occupied by Ca2+ ions showed a high critical current densityJc in the order of 105A/cm2.We clarified chemical bond nature of as-grown Bi-based superconducting whiskers by X-ray photoemission spectroscopy XPS. Therefore, we confirmed that Ca2+ ions actually occupied Sr2+ sites by existence of another XPS peak at higher binding energy of each Ca-2p3/2,1/2 XPS spectrum.To clarify an origin of the Jc enhancement, we observed local structures of Bi-based superconducting whiskers by high-resolution transmission electron microscopy. Therefore, we found that strains which were come from a smaller ion radius of Ca2+ ion compared with one of Sr2+ ion. The strains affected a lattice mismatch between CuO2 plane and SrO plane in Bi-based superconductor and they forced to change modulation lengths in some unit cells. These strains seem to have worked seffective pinning centers in Bi-based superconducting whiskers.