Electronic states of high-Tc cuprate superconductors as studied by the use of the composite-operator approach.

Abstract

The doping dependence of the electronic states in cuprate superconductors is studied by use of the p-d-mixing model. The p-d hopping under strong correlation is treated by introducing electronic excitations associated with Cu-O 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 electronic states with carrier doping are understood by mixing and decays among those composite electronic excitations. By hole doping, transfer of the density of states to the Fermi level is induced from both the upper Hubbard band and the bottom of the valence band. It is shown that the crossover from a highly correlated electron band to a simple mixing band may be understood by allowing an interchange of energy positions between the upper Hubbard and composite excitation levels. The doping dependence of intrasite charge and spin fluctuations at the Cu site is also discussed. Specially, a prominent intensity transfer to a low-energy region with carrier doping is found to result in the charge fluctuation.