Studies on inhomogeneous electronic structures in high J_c ceramics superconductors

为了阐明导致高T_c超导体中高临界电流密度（J_c）的非均匀电子结构，我们利用扫描隧道显微镜/能谱（STM/STS）研究了具有实用价值的高J_c材料NdBaCuO和掺Pb的BiSrCaCuO。超导带隙结构为2Δ <$50 meV，与文献数据一致。背景电导的形状是相当平坦的，相反的Y123表现出抛物线的隧道数据的大部分。在解理的表面上，我们经常看到窄的空间区域与间隙内的电导峰，可能源于准粒子散射的原子尺度的缺陷，如氧空位。这些区域被认为是强钉扎中心。在掺Pb的BiSrCaCuO中，我们已经成功地观察到了贫Pb和富Pb区域的相分离。从液氮温度附近的STM/STS测量结果可以看出，前者比后者具有更高的T_c。此外，两个区域之间的界面相当尖锐，约为几nm。因此，预期界面有效地捕获涡流，特别是在较高温度下。

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.