Direct Observation of Cu-O Plane by STM and Local Pair-breaking Effect on High-T_c Superconductor

用扫描隧道显微镜观察了Bi2212的Cu-O面的原子像。过掺杂样品的Cu-O面的原子像具有交叉条纹的特征，这意味着载流子的轨道将是Cu-3d和O-2p轨道强烈杂化的结果。另一方面，欠掺杂(UD)样品的输运和磁性表明，随着掺杂水平的降低，Cu-3d和O-2p轨道的杂化将受到抑制。因此，我们可以预期，STM的铜氧原子像在UD区变成了一个简单的圆圈阵列。然而，由于我们未能制备出高质量的UD单晶，我们无法证实原子STM图像的这种变化。在本研究中，我们还研究了非磁性的锌离子和磁性的镍离子对La214超导电性的杂质效应。我们发现，在铜3d自旋网络上形成的磁关联受到锌离子干扰的区域，超导电性受到局部抑制。这就是为什么非磁性的锌离子比磁性的镍离子具有更强的局域断裂效应的原因，并暗示了Cu3d自旋网络将在高T_c超导电性中起到关键作用。研究还发现，在平均场温T^*(~2Δ_O/4.3K_B)附近，赝隙开始逐渐发展，并且容易被掺杂杂质抑制。T^*对杂质浓度的依赖关系与T_c非常相似，表明赝隙与高T_c超导电性密切相关。

We observed the atomic image of the Cu-O plane of Bi2212 using STM technique. The atomic image of the Cu-O plane for over-doped samples is characterized by cross-striped pattern, implying that the orbit of carriers will result from a strong hybridization of Cu-3d and O-2p orbits. On the other hand, transport and magnetic properties of under-doped (UD) samples indicate that the hybridization of Cu-3d and O-2p orbits will be rather suppressed as doping level is reduced. Therefore, we can expect that the Cu-O atomic image of STM is changed into a simple array of circles in the UD region. However, we could not confirm such a change of the atomic STM image because we failed to prepare UD single crystals of high quality. In the present study, we also studied impurity effects of nonmagnetic Zn- and magnetic Ni-ions on the superconductivity of La214. We found that the superconductivity is locally suppressed over the region where magnetic correlation developed on the Cu 3d-spin network is disturbed by Zn ions. This is the reason why nonmagnetic Zn-ions cause strong local pair-breaking effect compared with magnetic Ni-ions, and implies that the Cu 3d-spin network will play a crucial role in causing the high-Tc superconductivity. It was also found that the pseudo-gap state begins to develop gradually around the mean-field temperature T^* (〜2Δ_O/4.3k_B) and is easily suppressed by doping impurities. The dependence of T^* on impurity concentration is very similar to that of Tc, indicating that the pseudo-gap is closely related with the high-Tc superconductivity, as expected.