THEORETICAL STUDY FOR MAGNETISM AND CONDUCTIVITY, AND OPTICAL PROPERTIES IN MAGNETIC SEMICONDUCTORS

应用动态相干势近似的简单模型，系统地研究了A_<1-x> mn_xb型稀释磁性半导体（DMS’s）的载流子态。对三种典型的DMS进行了模型计算：在没有非磁势的情况下具有强而中等的交换相互作用，以及在具有强吸引非磁势的情况下具有中等的交换相互作用。当交换作用足够强时，磁性杂质带从主带分裂。磁性杂质带中的载流子主要停留在磁性位上，载流子自旋与局域自旋之间的耦合很强。载流子在磁性位点之间的跳跃通过类似双交换（DE）的机制产生铁磁性。我们研究了类de机制在DMS中运行的条件。结果表明，磁性位点的非磁性吸引势通过显著增强交换相互作用的效果，促进了磁性杂质带的形成，使类de机制得以运行。用常规参数研究了Ga_<1-x>Mn_xAs中的载流子态。结果表明，带尾中杂质态的载流子通过类de机制产生了铁磁性。

Applying the dynamical coherent potential approximation to a simple model, we have systematically studied the carrier states in A_<1-x>Mn_xB-type diluted magnetic semiconductors (DMS's). The model calculation was performed for three typical cases of DMS's : The cases with strong and moderate exchange interactions in the absence of nonmagnetic potentials, and the case with strong attractive nonmagnetic potentials in addition to moderate exchange interaction. When the exchange interaction is sufficiently strong, magnetic impurity bands split from the host band. Carriers in the magnetic impurity band mainly stay at magnetic sites, and coupling between the carrier spin and the localized spin is very strong. The hopping of the carriers among the magnetic sites causes ferromagnetism through a double-exchange (DE)-like mechanism. We have investigated the condition for the DE-like mechanism to operate in DMS's. The result reveals that the nonmagnetic attractive potential at the magnetic site assists the formation of the magnetic impurity band and makes the DE-like mechanism operative by substantially enhancing the effect of the exchange interaction. Using conventional parameters we have studied the carrier states in Ga_<1-x>Mn_xAs. The result shows that the ferromagnetism is caused through the DE-like mechanism by the carriers in the bandtail originating from the impurity states.