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

本文应用动力学相干势近似，系统地研究了A_<1-x>Mn_xB型稀磁半导体（DMS’s）的载流子态。对DMS的三种典型情况进行了模型计算：在没有交换势的情况下具有强和中等交换相互作用的情况，以及除了中等交换相互作用之外具有强吸引力的交换势的情况。当交换相互作用足够强时，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置，并且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）的机制引起铁磁性。我们已经调查的条件DE样机制在DMS的操作。结果表明，磁位的磁吸引势有助于磁性杂质带的形成，并通过显著增强交换相互作用的作用，使类DE机制起作用.用常规参数研究了Ga_ Mn_xAs中的载流子态<1-x>。结果表明，铁磁性是由杂质态产生的尾带载流子通过类DE机制引起的。

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.