While most of crystalline wide gap oxides are both stoichiometric and insulating, a handful of them including ZnO and In 2 O 3 are naturally anion-deficient and electron conductors. Even fewer of the oxides are naturally cation-deficient and hole conductors, the arch-type of which is Cu 2 O. Based on first principles calculation of equilibrium nonstoichiometry and defect stability, we explain why the Cu ͑I͒ ͑d 10 ͒ oxide-based materials are both p-type and naturally cation-deficient, and why cation vacancies lead to delocalized, conductive states, whereas in other oxides ͑e.g., ZnO and MgO͒, they lead to localized, nonconductive states.

Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and 'ionic radii' which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms--that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence--such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization--that have often been interpreted as literal charge transfer are instead a consequence of the negative-feedback charge regulation.

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The well-known "band-gap" problem in approximate density functionals is manifested mainly in an overly low energy of the conduction band ͑CB͒. As a consequence, the localized gap states of 3d impurities states in wide-gap oxides such as ZnO occur often incorrectly as resonances inside the CB, leading to a spurious transfer of electrons from the impurity state into the CB of the host, and to a physically misleading description of the magnetic 3d-3d interactions. A correct description requires that the magnetic coupling of the impurity pairs be self-consistently determined in the presence of a correctly positioned CB ͑with respect to the 3d states͒, which we achieve here through the addition of empirical nonlocal external potentials to the standard density functional Hamiltonian. After this correction, both Co and Cr form occupied localized states in the gap and empty resonances low inside the CB. In otherwise undoped ZnO, Co and Cr remain paramagnetic, but electron-doping instigates strong ferromagnetic coupling when the resonant states become partially occupied.

3d transition impurities in wide-gap oxides may function as donor/acceptor defects to modify carrier concentrations, and as magnetic elements to induce collective magnetism. Previous first-principles calculations have been crippled by the LDA error, where the occupation of the 3d-induced levels is incorrect due to spurious charge spilling into the misrepresented host conduction band, and have only considered magnetism and carrier doping separately. We employ a band-structure-corrected theory, and present simultaneously the chemical trends for electronic properties, carrier doping, and magnetism along the series of 3d 1 -3d 8 transitionmetal impurities in the representative wide-gap oxide hosts In 2 O 3 and ZnO. We find that most 3d impurities in In 2 O 3 are amphoteric, whereas in ZnO, the early 3d's ͑Sc, Ti, and V͒ are shallow donors, and only the late 3d's ͑Co and Ni͒ have acceptor transitions. Long-range ferromagnetic interactions emerge due to partial filling of 3d resonances inside the conduction band and, in general, require electron doping from additional sources.

The magnetization and hole distribution of Mn clusters in (Ga,Mn)As are investigated by allelectron total energy calculations using the projector augmented wave method within the densityfunctional formalism. It is found that the energetically most favorable clusters consist of Mn atoms surrounding one center As atom. As the Mn cluster grows the hole band at the Fermi level splits increasingly and the hole distribution gets increasingly localized at the center As atom. The hole distribution at large distances from the cluster does not depend significantly on the cluster size. As a consequence, the spin-flip energy differences of distant clusters are essentially independent of the cluster size. The Curie temperature TC is estimated directly from these spin-flip energies in the mean field approximation. When clusters are present estimated TC values are around 250 K independent of Mn concentration whereas for a uniform Mn distribution we estimate a TC of about 600 K.

Abstract. -The interplay between clustering and exchange coupling in magnetic semiconductors for the prototype (Ga1−x, Mnx)As with manganese concentrations x of 1/16 and 1/32 in the interesting experimental range is investigated. For x ∼ 6%, when all possible arrangements of two atoms within a large supercell are considered, the clustering of Mn atoms at nearest-neighbour Ga sites is energetically preferred. As shown by spin density analysis, this minimum energy configuration localizes further one hole and reduces the effective charge carrier concentration. Also the exchange coupling constant increases to a value corresponding to lower Mn concentrations with decreasing inter Mn distance.Including spin information into semiconductor electronics has enormous potential for new applications (see Ref.[1] for a review on magnetoelectronics). An interesting situation arises when the carriers responsible for metallic behaviour are of the same spin, in the so called half-metals [2]. Within this class of materials, the magnetic semiconductors are particularly challenging, not only because they can replace magnetic elements in storage media, but because they can be used as spin injectors into normal semiconductors [3,4] for completely novel applications. In the field of magnetic semiconductors a new chapter has been opened by the III-V diluted magnetic semiconductors (DMS), especially with (Ga, Mn)As.Experimentally (Ga, Mn)As can be manufactured using low-temperature molecular beam epitaxy (LT-MBE) [5,6,7,8] and its magnetic properties have been measured with numerous techniques [5,6,7,8,9,10,11]. (Ga,Mn)As is ferromagnetic (see e.g. Refs. [6, 7, 8]), with a Curie temperature as large as 110 K in the Mn concentration range between 5% and 10%. In addition, the temporal evolution of the magnetization during annealing shows that first ferromagnetism is enhanced and then reduced, [11], suggestive of clustering processes.In this Letter, we first survey the current state of theoretical research in the field. To overcome all the possible sources of errors a new benchmark is needed. We then present calculations for magnetic semiconductors, studying (Ga 1−x , Mn x )As in different magnetic configurations. In particular, we assess the role of Mn clustering in the lattice (see Fig.1 describing the considered cases). Finally, the clustering results drive us to reexamine the findings of previous calculations that address the role of the exchange coupling parameter within mean-field theories.The ferromagnetism (FM) in (Ga,Mn)As is mediated by holes that are antiferromagnetically (AFM) coupled to the Mn. Theoretical studies using state-of-the-art computational c EDP Sciences

The effect of microscopic Mn cluster distribution on the Curie temperature (Tc) is studied using density-functional calculations. We find that the calculated Tc depends crucially on the microscopic cluster distribution, which can explain the abnormally large variations in experimental Tc values from a few K to well above room temperature. The partially dimerized Mn_2-Mn_1 distribution is found to give the highest Tc > 500 K, and in general, the presence of the Mn_2 dimer has a tendency to enhance Tc. The lowest Tc values close to zero are obtained for the Mn_4-Mn_1 and Mn_4-Mn_3 distributions.Comment: To appear in Applied Phyiscs Letter