Theoretical Study on Charge Fluctuation in Strongly Correlated Electrons

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The main aim of this research project is, as one of the clues to clarify the mechanism for obtaining the high-T_c superconductor, to investigate roles of charge fluctuation in the d-p model which is believed to describe the strongly correlated electrons in the CuO_2 2-dimensional electron systems well.1. Firstly we applyed the 1/N-expansion theory to study propertie of the impurity Anderson model and the lattice Anderson model which have been known to some extent, have clarified the roles of the charge fluctnation and confirmed the usefullness of the 1/N-expansion theory.2. Secondly we employed the 1/N-expansion theory to investigate properties of the d-p model and have found the charge fluctuation to play important roles for determining the properties of the high-T_c superconductors as follows :(a)At lower temperatures(T<T_0)where T_0 is the so-called coherence temperatute, hole doping creates some new stable quasiparticle states inside the charge transfer energy gap(called as ingap states)and the system behaves as a Fermi liquid. By denoting the band width of the ingap states omega_0, T_0 is given by T_0 (〕SY.simeq.〔) 0.1omega_0.(b)At intermediate temperatures, (T_0<T<omega), the system behaves as a marginal Fermi liquid, where the quasi-particle life time is proportional to 1/T and the magnetic susceptibility shows cross-over from the Pauli Paramagnetism to the Curie law.(c)The Coulomb repulsion between electrons at the nearest Cu and O sites enhances the charge fluctuation, the exchange interaction between electrons at the nearest Cu sites stabilizes the antiferromagnetic order in the ingap states and the direct transfer between electrons at the nearest O sites may modify the properties of the systems significantly.