A theoretical study of the effects of disorder on the Mn-Mn exchange interactions for Ga1-xMn xAs diluted magnetic semiconductors is presented. The disorder is intrinsically considered in the calculations, which are performed using an ab initio total energy density-functional approach, for a 128 atoms supercell, and by considering a variety of configurations with 2, 3 and 4 Mn atoms. Results are obtained for the effective J$^{Mn-Mn}_n$, from first (n = 1) all the way up to sixth (n = 6) neighbors via a Heisenberg Hamiltonian used to map the magnetic excitations from ab initio total energy calculations. One then obtains a clear dependence in the magnitudes of the J$^{Mn-Mn}_n$ with the Mn concentration x. Moreover, we show that, in the case of fixed Mn concentration, configurational disorder and/or clustering effects lead to large dispersions in the Mn-Mn exchange interactions. Also, calculations for the ground-state total energies for several configurations suggest that a proper consideration of disorder is needed when one is interested in treating temperature and annealing effects.

We describe a procedure to calculate charge transport properties across a nanosystem. This scheme is based on a Green's Function formalism to treat a non-equilibrium problem, coupled to the Density Functional Theory to describe the electronic structure. As an illustration, we perform calculations for the charge transport across a (5,5) carbon nanotube with a vacancy.

We investigated the interaction of As impurities with a 90º partial dislocation in Si. The calculations show that As atoms segregate to the dislocation core. The most stable site for the As atom is at the stacking fault side, which is favorable by 0.26 eV as compared to an As atom in a crystalline position. There is no charge transfer from the As impurity to the dislocation core when the impurity is far from it. The segregation of the impurity can be understood mainly due to structural effects. This result let us conclude that the experimentally observed negatively charged dislocations are due to impurities trapping at the core of the dislocation and not charge transfer from impurities away from the dislocation.

By combining ab initio methods and interatomic potentials, we investigated the electronic and the structural properties of amorphous silicon nitride. Our results show the trends on the electronic band structure of SiNx (0.5 < x < 1.8) as the nitrogen content changes, and are in good agreement with the experimental data.Hydrogenation of the systems was found to considerably reduce the number of energy levels in the gap. The energy levels which appeared in the materials band gap were compared to the electrically active centers, which have been identified by electron-paramagnetic resonance experiments.