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

对于磁学来说，最重要的问题之一是建立一个既适用于局部磁学又适用于流动电子磁学的统一理论。提出了1/d展开法，其中d为空间维数。这既适用于局部力矩磁力，也适用于流动电子磁力。在最佳单点近似（SSA）中可以正确地考虑局域量子自旋涨落。SSA相当于求解映射的Anderson模型，使其在映射条件下自洽确定。这意味着近藤效应不仅在稀磁合金中，而且在周期性强相关电子系统中都起着重要的作用。从SSA开始，可以扰动地处理场间效应。用1/d展开法研究了各种问题。(1)研究了磁性结构在正弦和螺旋结构之间的交叉。(2)除模-模耦合机制外，居里-魏斯定律在1/d内还存在另一种更高阶的机制：磁场对温度的依赖，在1/d内居首位。(3)提出了铜氧化物超导体高温超导的机理；Mott跃迁附近重准粒子之间形成的dx_<x2-y2>波库珀对。主要的配对相互作用是最近邻居之间的超交换相互作用。(4)研究了一种新型的磁交换相互作用。它是由准粒子在自旋通道中对激发的虚交换引起的。这种交换相互作用的强度大约是准粒子的带宽。(5)本文提出了CeRu_2Si_2的变磁跃迁主要是由这种交换作用引起的。

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.