Theoretical Study of Magnetism and Transport Properties in Diluted Magnetic Semiconductors

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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