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The formation and binding energies, the ionization levels, the structures, and the local vibrations of O i , O 2i , O 3i , VO, VO 2 , and V 2 O (Vϭvacancy͒ in silicon are calculated using a self-consistent total-energy pseudopotential method. The most important results are as follows: The ionization levels and associated structures are given for VO and V 2 O as well as the local vibration modes for the negative charge states of VO.

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.

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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

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A general kinetic model based on accurate density-functional-theoretic total-energy calculations is introduced to describe the aggregation kinetics of oxygen-related thermal double donors (TDD's) in silicon. The calculated kinetics, which incorporates the reactions of associations, dissociations, and isomerizations of all relevant oxygen complexes, is in agreement with experimental annealing studies. The aggregation of TDD's takes place through parallel-consecutive reactions where both mobile oxygen dimers and fast migrating chainlike TDD's capture interstitial oxygen atoms.

We have analyzed a simple model of a periodically modulated magnetic system. The phase diagram includes multiple phase transitions between commensurate phases, similar to those observed in CeSb. We argue that our model, and CeSb, are possible candidates for the "devil's stairs" behavior.