Spin and charge fluctuation and electronic state in high-tc superconductors

By use of the p-d mixing model, we investigate doping dependence of the electronic state and spin and charge fluctuations in high temperature superconductivity. The p-d hopping under strong correlations is treated by introducing electronic excitations associated with Cu-O covalent bonds, which are described by use of composite operators composed of p electrons and neighboring d electron spin and charge fluctuations. It is shown that changes of the electronic state with carrier doping are understood by mixing and decays among those composite excitations. The local antiferromagnetic correlation and the motion of electrons are strongly correlated in the sense that the local antiferromagnetic order works to narrow the band width, while the motion of electrons near the Fermi level suppress the local antiferromagnetic correlation by disturbing the local spin order. An indication of low energy excitations in charge fluctuation is pointed out, which extends about the conduction band width in the moderately doped metallic region.