Electron Self-Organizations and Transport Phenomena in High-Tc Cuprate Superconductors

点击翻译按钮获取中文摘要

It is recognized that some sort of electron self-organization occurs in high-Tc cuprate superconductors due to strong electron correlations. Also, it seems likely that in cuprates the electron self-organization phenomena lie behind their peculiar electronic states such as unusual insulating normal state under high magnetic fields or intrinsic inhomogeneity of the superconducting gap. This project has clarified various aspects of the electron self-organization phenomena in cuprates through experiments using high-quality single crystals, in particular their transport properties measurements under extreme conditions. Principal results are the following:1. We discovered the existence of "magic doping fractions" for charge ordering in La_2-X_Sr_xCuO_4, and showed that a checkerboard model consistently explains the observed fractions.2. Extensive measurements of ultra-low-temperature thermal conductivity of YBa_2Cu_3O_y, La_2-x_Sr-xCuO_4, and Bi_2Sr_2CaCu_xO_<8+δ> revealed a breakdown of the "universal thermal conductivity", which is generically expected for d-wave BCS superconductors. This breakdown is apparently related to the peculiar superconducting gap inhomogeneity in cuprates.3. Through the ultra-low-temperature measurements of transport properties of nonsuperconducting YBa_2Cu_3O_y, where a self-organized resistivity anisotropy is known to exist, we elucidated that the electron self-organized state is inherently insulating when it is nonsuperconducting. Moreover, we found that this insulating state is characterized by the peculiar log(1/T) resistivity divergence.4. We elucidated that the charge transfer energy in the CuO_2 plane can be obtained from the temperature dependence of the Hall coefficient measured up to 1000 K. Furthermore, we showed that the Hall coefficient data can be used for disentangling the phase-separated nature of La_2-x_Sr_xCuO_4 in the superconducting doping regime.