We study optical properties of a single band model for (III,Mn)V diluted magnetic semiconductors (DMS), treating the impurity spins fully quantum mechanically in the coherent potential approximation (CPA) Our CPA results show an interesting dependence of the optical conductivity on Kondo coupling, carrier density, nonmagnetic potential, and temperature We find that by taking into account the spin–flip effect, the calculated conductivity is in reasonable agreement with the ones obtained by Monte-Carlo simulations and experiments

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Optical properties of diluted magnetic semiconductors in coherent potential approximation

The theoretical study of magnetic semiconductors using the dynamical coherent potential approximation (dynamical CPA) is briefly reviewed. First, we give the results for ferromagnetic semiconductors (FMSs) such as EuO and EuS by applying the dynamical CPA to the s-f model. Next, applying the dynamical CPA to a simple model for A1−xMnxB-type diluted magnetic semiconductors (DMSs), we show the results for three typical cases to clarify the nature and properties of the carrier states in DMSs. On the basis of this model, we discuss the difference in the optical band edges between II-V DMSs and III-V-based DMSs, and show that two types of ferromagnetism can occur in DMSs when carriers are introduced. The carrier-induced ferromagnetism of Ga1−xMnxAs is ascribed to a double-exchange (DE)-like mechanism realized in the magnetic impurity band/or in the band tail.

Carrier States in Ferromagnetic Semiconductors and Diluted Magnetic Semiconductors—Coherent Potential Approach—

The magnetic properties over a wide range of temperatures in diluted magnetic semiconductors (DMSs) are discussed in the framework of the Kondo lattice model with magnetic impurity disorder. In the approach of dynamical mean-field theory, a set of self-consistent equations has been derived to specify single-particle Green functions. Analytical expressions of the static magnetic susceptibility and the ${B}_{1g}$ channel Raman response are then delivered. Inspecting for signatures of an itinerant carrier density of states and magnetization, we have identified a stable ferromagnetic (FM) state at low temperature. With increasing temperature, thermal fluctuations diminish the ordered state and the system favors a paramagnetic (PM) phase. Tracking and tracing different model parameters, the FM-PM transition phase diagram has been constructed. Spin fluctuations in the PM state have been pointed out in the behaviors of the self-energies, the static magnetic susceptibility, and the Raman scattering. Analyzing the properties of the ${B}_{1g}$ channel Raman response, we have attributed a formation of magnetic polarons in the PM state. The magnetic polaron scenario in a wide range of temperatures has been then intensively discussed. For the first time, both the static magnetic susceptibility and the ${B}_{1g}$ Raman response have been combined together to discuss the magnetic properties in DMSs compactly based on the dynamical mean-field approach.

Ferromagnetic transition and spin fluctuations in diluted magnetic semiconductors

The antiferromagnetic-ferromagnetic competition in a diluted magnetic semiconductor is discussed. In a virtual-crystal approximation, the active magnetic ions in the system are assumed to distribute homogeneously, and the quantum magnetic correlations are described in the Kondo lattice model involving local disorder. In the framework of the dynamical mean-field theory, we deliver the signatures of the static magnetic susceptibility function and the ${B}_{1g}$ channel Raman response adapting to the model. At low temperatures, one finds the antiferromagnetic instability against the ferromagnetic state when the impurity band is nearly filled. For a completely filled impurity band system, the antiferromagnetic state is stabilized in the case of sufficiently large magnetic exchanges. The effect of the thermal fluctuations on the phase competition is also discussed. Analyzing the ${B}_{1g}$ channel Raman spectrum attributes a competition between the short-range magnetic order and the short-wavelength excitation in the filled impurity band paramagnetic state.

Magnetic features near filled impurity band in diluted magnetic semiconductors

Theory of diluted magnetic semiconductors: A minimal model

We investigated the transport and optical properties of diluted magnetic semiconductors theoretically by using a simple model where carriers move in a single band. In this model the carrier feels a nonmagnetic potential at a magnetic impurity site, and its spin interacts with the localized spins of the magnetic impurities through exchange interactions. The electronic states of a carrier were calculated by using the coherent potential approximation (CPA). The magnetism was investigated by minimizing the free-energy and the electrical conductivity was calculated by using the Kubo formula. We examined the results in several typical cases which correspond to (Ga1−xMn
 x
 )As with x = 0.05.

Kazushi Kagami

Papers

Transport and optical properties of diluted magnetic semiconductors

Theory of diluted magnetic semiconductors: A minimal model