Theoretical study on magnetism and superconductivity in almost lo-calized strongly correlated elecron systems

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One of the most important issues for magnetism is to construct a unified theory that is applicable to both local-moment magnetism and itinerant electron magnetism. The 1/d expansion methof has been developed, with d being the spatial dimensionality. This is apllicable to both local-moment magnetism and itinerant electron magnetism. Local quantuam spin fluctuations can be correctly considered in the best single-site approximation (SSA). The SSA is equivalent to solving a mapped Anderson model to be self-consistently determined form a maaping condition. This means that the Kondo effect plays a significant and essential role in not only dilute magnetic alloys but also periodical strongly correlated electron systems. Intersite effects can be perturbatively treated starting from the SSA.Various issues have been studied by the 1/d expansion method. (1) The corssover of magnetic structures between sinusoidal and helical stuructures has been studied. (2) There is another mechanism of the Curie-Weiss law for itinerant electron magnetism besides the mode-mode coupling mechanism, which is of higher in 1/d : the temperature dependence of the magnetic mean field, which is of leading order in l/d. (3) A mechanism of high-temperature superconductivity in cuprate-oxide superconductors has been proposed ; the formation of dx_<x2-y2> wave Cooper pairs between heavy quasiparticles in the vicinity, of the Mott transtion. The main pairing interaction is the superexchange interaction between nearest neighbors. (4) A new type of magnetic exchange interaction has been studied. It is caused by the virtual exchange of pair excitatiolns of quasiparticles in spin channels, has been studied. The strength of this exchange interaction is about the bandwidth of quisipartgicles. (5) It has been proposed that a metamagnetic transition in CeRu_2Si_2 is mainly caused by this exchange interaction.