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Theoretical Study of Magnetism and Transport Properties in Diluted Magnetic Semiconductors

稀磁半导体磁性和输运特性的理论研究

基本信息

批准号：
17540339
负责人：
KUBO Kenn
金额：
$ 2.18万
依托单位：
Aoyama Gakuin University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2005
资助国家：
日本
起止时间：
2005 至 2006
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-17540339/
关键词：
diluted magnetic semiconductors carrier induced ferromagnetism optical conductivity minimal model coherent potential approximation partial line graph flat energy band Hubbard model 強磁性引力ポテンシャル部分ライングラフ CPA フラットバンド部分強磁性輸送現象 /

项目摘要

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

1)提出了一个包含稀磁半导体本质特征的最简单模型，我们称之为“最小模型”。该模型是用具有局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。具有吸引力的库仑势以及载流子自旋和局域自旋之间的交换相互作用作用于杂质位置的载流子。我们利用相干势近似(CPA)研究了该模型。结果发现，磁场的特征行为可以分为强耦合和弱耦合两种类型。在弱耦合区，虽然居里温度较低，但系统可用平均场图像描述，并且在较宽的载流子密度范围内实现了铁磁性。在强耦合区形成杂质带，杂质带中载流子的自旋与局域…紧密结合更多的d旋转。因此，这种类似于锰氧化物中双交换作用的机理是可行的。结果表明，尽管居里温度较高，但只有当载流子浓度小于杂质浓度时，才能实现铁磁性。即使交换耦合极大地增加，居里温度也会饱和。我们还研究了它的光学导电性。计算结果表明，随着与杂质耦合程度的增加，电导率的Drude峰逐渐减小并最终消失，在一个有限的频率处出现另一个峰。这与实验观测结果定性一致。直接电导率的计算结果随参数的不同呈现出不同的温度依赖关系，但与实验结果不能很好地吻合。2)通过数值模拟，我们考察了在最小模型中被假定为接触型的库仑吸引势范围的影响。虽然我们不能模拟足够大的系统，但我们对一个有100个位置的星系团的结果表明，更长的势能范围导致更宽的杂质带。3)我们提出了一类称为“广义部分线图”的晶格结构，紧束缚带结构实现了平坦的能带。我们用有限团簇的精确对角化方法研究了几个GPLG算例的Hubbard模型中的磁性。结果表明，具有GPLG晶格结构的体系有望实现完全和部分亚铁磁性、亚铁磁性和反铁磁性。较少