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

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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.