Studies on inhomogeneous electronic structures in high J_c ceramics superconductors

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In order to clarify the inhomogeneous electronic structures responsible for high critical current densities (J_c) in high T_c superconductors, we have performed scanning tunneling microscopy/spectroscopy (STM/STS) on NdBaCuO and Pb-doped BiSrCaCuO, being promising candidates for practical high J_c materials, at low temperatures below T_cIn NdBaCuO, we reproducibly found a clear. superconducting gap structure with 2Δ〜50 meV, which is in agreement with the literature data. The shape of the background conductance is fairly flat, contrary to the most of the tunneling data of the Y123 exhibiting the parabolic one. On the as-cleaved surfaces, we have frequently seen narrow spatial regions with in-gap conductance peaks, possibly originating from quasi-particle scattering by atomic scale defects, such as oxygen vacancy. These regions are thought to act as strong pinning centers.In Pb-doped BiSrCaCuO, we have succeeded in visualizing phase separation into Pb-poor and Pb-rich regions. From the STM/STS measurements around liquid nitrogen temperature, it has been revealed that the former has higher T_c than the latter. In addition, the interface between two regions are quite sharp, 〜several nm. Therefore, it is expected that the interfaces effectively trap vortices particularly at higher temperatures.