Theoretical Study of Magnetism and Transport Properties in Diluted Magnetic Semiconductors

1)我们提出了包含稀释磁性半导体基本特征的最简单模型，我们称之为“最小模型”。该模型是一个用具有定域自旋的磁性杂质随机取代的晶格模型。载流子在晶格点的轨道间跳跃。有吸引力的库仑势以及载流子自旋和定域自旋之间的交换相互作用作用于杂质位点的载流子上。我们利用相干势近似（CPA）对该模型进行了研究。结果表明，磁性的特征行为可分为强耦合和弱耦合两类。在弱耦合状态下，系统可以用平均场图来描述，虽然居里温度相对较低，但在载流子密度的宽区域内实现了铁磁性。在强耦合区形成杂质带，杂质带中载流子的自旋与局域自旋紧密结合。因此，类似于锰氧化物中双交换作用的机理是可行的。因此，尽管居里温度较高，但只有当载流子密度小于杂质浓度时，才能实现铁磁性。即使交换耦合大大增加，居里温度也会饱和。我们还研究了光学导电性。计算结果表明，当与杂质的耦合增加时，电导率的德鲁德峰逐渐减小并最终消失，在有限频率处出现另一个峰值。这与实验观察结果在质量上是一致的。直接电导率的计算结果在温度依赖关系中表现出不同的行为，但我们得到的结果与实验结果不太一致。2)通过数值模拟，考察了杂质对库仑吸引势范围的影响，在最小模型中假设杂质为接触型。虽然我们不能模拟足够大的系统，但我们对100个点的团簇的结果表明，更长的电位范围导致更宽的杂质带。结果表明，长范围电势导致较高的居里温度。3)我们提出了一类称为“广义偏线图（GPLG）”的晶格结构，紧结合带结构在其上实现了平面带。利用有限簇的精确对角化方法，研究了GPLG的Hubbard模型的磁性。结果表明，具有GPLG晶格结构的体系是实现完全铁磁性和部分铁磁性、铁磁性和反铁磁性的有希望的候选者。少

1) We proposed a simplest model which contains essential features of the diluted magnetic semiconductors, which we call "minimal model". This model is a lattice model with randomly substituted with magnetic impurities with localized spins. Carriers hop among orbitals at lattice sites. An attractive Coulomb potential as well as an exchange interaction between a carrier spin and a localized spin exerts on a carrier at an impurity site. We investigated the model by use of the coherent potential approximation(CPA). As a result we found that the characteristic behavior of the magnetism can be categorized into the strong coupling or the weak coupling regime. In the weak coupling regime, the system can be described by the mean field picture and the ferromagnetism is realized in a wide region of carrier density though the Curie temperature is relatively low. In the strong coupling regime impurity bands are formed and the spins of carriers in the impurity bands are tightly bound to the localize … More d spins. Therefore the mechanism similar to the double-exchange interaction in manganum oxides is operative. As a result the ferromagnetism is realized only when the carrier density is less than the impurity concentration though the Curie temperature is relatively high. The Curie temperature saturates even the exchange coupling is increased extremely. We also studied the optical conductivity. The calculated results show that the Drude peak of the conductivity diminishes and finally vanishes when the coupling with the impurities is increased and another peak appears at a finite frequency. This agrees qualitatively with the experimental observations. The results on the direct conductivity show variety of behaviors in the temperature dependence according to the values of parameters, though we could not obtain good agreement with experimental results.2) Using numerical simulations, we examined the effect of the range of the Coulomb attractive potential due to impurities, which is assumed to be of contact-type in the minimal model. Though we could not simulate sufficiently large systems, our result for a cluster with 100 sites showed that the longer range of the potential leads to the broader impurity band. The result suggests that the long ranged potential leads to the higher Curie temperature.3) We proposed a class of lattice structures called "generalized partial line graph(GPLG)", on which tight-binding band structures realize flat bands. We investigated the magnetism in the Hubbard model on several examples of GPLG by use of exact diagonalization of finite clusters. Obtained results showed that the systems with GPLG lattice structures are promising candidates for realizing complete and partial ferromagnetism, ferrimagnetism and antiferromagnetism. Less