Magnetic and transport properties of the double-exchange systems at interfaces and surfaces

1)建立了与经典自由度相互作用的电子系统的多项式展开蒙特卡罗方法。应用这种新方法，我们研究了广义双交换模型，以探讨锰氧化物中巨磁电阻的来源。我们详细研究了电子-声子耦合、轨道简并以及随机性的影响，并阐明了所有这些影响都通过电荷波动导致了巨大的磁阻。2)引入了“最小模型”作为稀磁半导体系统最简化的有效模型。我们用CPA研究了模型的热力学。研究发现，弱耦合和强耦合条件下的模型在载流子诱导铁磁性特性上存在明显差异。在弱耦合区，在载流子密度n的大范围内出现铁磁性，而Cu…More rie温度Tc保持较低。然而，在强耦合体系中，在载流子密度n <x的有限区域内观察到铁磁有序，其中x为杂质浓度，tsi在n ~ x/2.3处达到最大值。对于AV2_O_4 (A=Zn, Mg, Cd)，我们澄清了轨道自由度在双相转变和复杂磁序中的起源和作用。研究了AV_2O_4 （A=Cd, Hg）在磁场作用下的巨大半磁化平台。该模型不仅解释了实验结果，而且还预测了四极有序自旋向列相等各种奇异相。我们还分析了AIV2O4的结构相变，并提出了七聚体簇中自旋单重态的形成。4)研究了带平面的部分线形图格上的Hubbard模型。由于强相关性，我们研究了磁性。结果，我们观察到不同的磁相，如部分铁磁性、铁磁性和反铁磁性。少

1) We developed the polynomial-expansion Monte Carlo method for electron systems which are interacting with classical degrees of freedom. By applying this new method, we studied generalized double-exchange models in order to investigate the origin of the colossal magnetoresistance in manganese oxides. We investigated the effects of electron-phonon couplings, orbital degeneracies as well as randomness in detail, and clarified that all of these effects contribute to the colossal magnetoresistance through charge fluctuations.2) We introduced the "Minimum model" as the most simplified effective model for dilute magnetic semiconductor system. We studied the thermodynamics of the model using CPA. It is found out a clear difference in the characteristic properties of the carrier-induced ferromagnetism between the model in the weak coupling regime and that in the strong coupling regime. In the weak coupling regime, ferromagnetism appears in the wide range of the carrier density n, while the Cu … More rie temperature Tc is kept low. In the strong coupling regime, however, ferromagnetic order is observed in a limited region of the carrier density n <x, where xis the impurity concentration, and Tcis maximum at n〜x/2.3) We studied spinel oxides which exhibit strong electron correlations and geometrical frustrations. Concerning AV2_O_4 (A=Zn, Mg, Cd), we clarified the origin and the role of orbital degrees of freedom in the double phase transition and complex magnetic orders. For AV_2O_4 (A=Cd, Hg), we studied the gigantic half-magnetization plateau under magnetic field. The model not only explains the experimental results but also predicts various exotic phases such as quadropole ordered spin nematic phase. We also analyzed the structural phase transition in AIV2O4, and proposed the formation of the spin singlet state in the Heptamer cluster.4) We studied the Hubbard model on the partial line graph lattice which exhibits flat bands. We investigated magnetisms due to strong correlations. As a result, we observed various magnetic phases such as partial ferromagnetism, ferrimagnetism and antiferromagnetism. Less