Study of electronic inhomogeneity and superconductivity in high-Tc superconductors by scanning tunneling microscopy.

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The aim of this program is to reveal atomic-scale electronic structures of high-Tc oxide superconductors by scanning tunneling microscopy (STM/STS) and to investigate the relationship between the superconductivity and the characteristic electronic structure, such as granular structure composed of the superconducting and pseudogap regions. In this study, we have observed atomic-scale electronic structures of YBa2Cu3Oy (YBCO) and Bi2-xPbxSr2CuOy (Pbx-Bi2201) single crystals with various carrier concentrations as well as macroscopic superconducting properties.1. YBa2Cu3Oy : (1) We have successfully prepared single crystals with different electronic coupling strength between adjacent CuO2 layers by controlling anneal conditions in nitrogen atmosphere and observed 2 to 3 dimensional transformation of the vortex system. (2) Effects of oxygen defects and impurities on the vortex structure and the electronic state have been studied macroscopically and microscopically. (3) One-dimensional elect … More ronic modulations on the BaO layer at low bias voltage have been observed along the Cu-O direction. These modulations are closely related to the electronic inhomogeneity induced by impurities and/or defects.2. Bi2-xPbxSr2CuOy : (1) The temperature and carrier concentration dependences of the superconducting transition temperature Tc and the pseudogap formation temperature T^* have been studied for well prepared Pb substituted Bi2201 single crystals. (2) It has been clarified that there exist two types of pseudogap ; one is related to a precursor effect of superconductivity and another to a strong electron correlation. The latter exist even in the overdoped region. (3) In STM images measured at high bias-voltages above 300 mV, a clear two-dimensional atomic arrangement, which consists of Bi and Pb atoms, has been observed. On the other hand, at low bias-voltages below 150 mV, it has been found that a contrasting density forms into bright patches of 1-3 nm diameters. These results are consistent with the electronic granularity observed in Bi2212. Less