Theoretical Study of Magnetism and Transport Properties in Diluted Magnetic Semiconductors

1)我们提出了一个包含稀磁半导体基本特征的最简单模型，我们称之为“最小模型”。该模型是一个晶格模型与随机取代磁性杂质与局域自旋。载流子在晶格点的轨道间跳跃。在杂质位置上的载流子受到库仑吸引势以及载流子自旋和局域自旋之间的交换相互作用的影响。我们利用相干势近似（CPA）研究了该模型.结果发现，磁性的特征行为可以分为强耦合和弱耦合两种。在弱耦合区，系统可以用平均场图描述，虽然居里温度较低，但在很宽的载流子密度范围内实现了铁磁性。在强耦合区，杂质带形成，杂质带中载流子的自旋与局域态紧密结合 ...更多信息 d旋转。因此，类似于锰氧化物中的双交换作用机制是有效的。结果表明，尽管居里温度相对较高，但只有当载流子浓度小于杂质浓度时才能实现铁磁性。当交换耦合极大增加时，居里温度也达到饱和。我们还研究了光导性。计算结果表明，随着与杂质耦合的增加，电导率的Drude峰逐渐减小，最后消失，并在有限频率处出现另一个峰。这与实验观察定性一致。直接电导率随温度的变化表现出不同的行为，但与实验结果并不一致。2）采用数值模拟的方法，研究了在最小模型中假定为接触型杂质的库仑吸引势范围对直接电导率的影响。虽然我们不能模拟足够大的系统，我们的结果与100个网站的集群表明，较长的范围内的潜在导致更广泛的杂质带。结果表明，长程势导致居里温度的升高。3）提出了一类称为“广义部分线图（GPLG）"的晶格结构，紧束缚带结构在该晶格结构上实现了平带。利用有限团簇的精确对角化方法研究了几个GPLG例子上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和部分铁磁性、亚铁磁性和反铁磁性的理想材料。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