A mean-field model of ferromagnetism mediated by delocalized or weakly localized holes in zinc-blende and wurzite diluted magnetic semiconductors is presented. The model takes into account strong spin-orbit and k⋅p couplings in the valence band as well as the influence of strain upon the hole density of states. Possible effects of disorder and carrier-carrier interactions, particularly near the metal-to-insulator transition, are discussed. A quantitative comparison between experimental and theoretical results for (Ga,Mn)As demonstrates that the theory describes the values of the Curie temperatures observed in the studied systems as well as explaining the directions of the easy axes and the magnitudes of the corresponding anisotropy fields as a function of biaxial strain. Furthermore, the model reproduces the unusual sign, magnitude, and temperature dependence of the magnetic circular dichroism in the spectral region of the fundamental absorption edge. Chemical trends and various suggestions concerning design of ferromagnetic semiconductor systems are described.

Surface core-level shifts for many semiconductor compounds are calculated in a tight-binding treatment, and with a local-charge-neutrality approximation. Both ideal and relaxed (110) surfaces are considered. Taking into account first-layer atomic displacements strongly modifies the values of calculated surface core-level shifts and provides a good agreement with experimental values.

The RKKY interaction between substitutional Mn local moments in GaAs is both spin-direction dependent and spatially anisotropic. In this paper we address the strength of these anisotropies using a semiphenomenological tight-binding model that treats the hybridization between Mn d-orbitals and As p-orbitals perturbatively and accounts realistically for its nonlocality. We show that valence-band spin-orbit coupling, exchange nonlocality, and band-structure anisotropy all play a role in determining the strength of these effects. We use the results to estimate the degree of ground-state magnetization suppression due to frustrating interactions between randomly located Mn ions and to comment on the relationships between different models of III-V diluted magnetic semiconductor ferromagnetism.

Long chapter on how transition metal behave in semiconductors.

We have calculated the electronic structure of neutral copper, nickel, cobalt, and iron impurities replacing gallium atoms in the gallium-arsenide lattice using the Xα scattered-wave method. Clusters of 17 atoms in a tetrahedral configuration are used to simulate the bulk and the locally perturbed crystal. The calculations were carried out to self-consistency and in the spin-polarized limit. The results indicate that the impurities studied yield acceptor energy levels deep in the crystal fundamental band gap, in agreement with experiment. The role played by the metal d states in the formation of the impurity centers is discussed and compared with the available experimental data.

While isovalent doping of GaAs (e.g., by In) leads to a repulsion between the solute atoms, two Cr, Mn, or Fe atoms in GaAs are found to have lower energy than the well-separated pair, and hence attract each other. The strong bonding interaction between levels with t2 symmetry on the transition metal (TM) atoms results in these atoms exhibiting a strong tendency to cluster. Using first-principles calculations, we show that this attraction is maximal for Cr, Mn, and Fe while it is minimal for V. The difference is attributed to the symmetry of the highest occupied levels. While the intention is to find possible choices of spintronic materials that show a reduced tendency to cluster, one finds that the conditions that minimize clustering tendencies also minimize the stabilization of the magnetic state.

We examine the intrinsic mechanism of ferromagnetism in dilute magnetic semiconductors by analyzing the trends in the electronic structure as the host is changed from GaN to GaSb, keeping the transition metal impurity fixed. In contrast with earlier interpretations which depended on the host semiconductor, it is found that a single mechanism is sufficient to explain the ferromagnetic stabilization energy for the entire series.

The recent development of MBE techniques for growth of III–V ferromagnetic semiconductors has created materials with exceptional promise in spintronics, that is, electronics that exploit carrier spin polarization. Among the most carefully studied of these materials is (Ga,Mn)As, in which meticulous optimization of growth techniques has led to reproducible materials properties and ferromagnetic transition temperatures well above 150 K. We review progress in the understanding of this particular material and efforts to address ferromagnetic semiconductors as a class. We then discuss proposals for how these materials might find applications in spintronics. Finally, we propose criteria that can be used to judge the potential utility of newly discovered ferromagnetic semiconductors, and we suggest guidelines that may be helpful in shaping the search for the ideal material.

The Coulomb interaction between localized electrons is shown to create a 'soft' gap in the density of states near the Fermi level. The new temperature dependence of the hopping DC conductivity is the most important manifestation of the gap. The form of the density of states within the gap is discussed.

In this paper we consider the properties of the disordered systems, when the mean free path of electrons is more or comparable with its wave length, i.e. some higher the localization edge. It is shown that the electron-electron interaction in this situation leads to an anomaly in the density of states near the Fermi level. We argue that this anomaly causes that of the tunnel resistance. The minimum in the temperature dependence of the resistivity in the disordered metals may also be due to a similar mechanism.