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Native defects in Si are of obvious importance in microelectronic device processing. Self-interstitials in particular are known to mediate, in many cases, anomalous impurity diffusion. Here we study the energetics and electronic structure of single, double, and triple self-interstitial clusters in crystalline Si in various charge states and geometries, providing extrinsic levels and binding-association-dissociation energies. We discuss the comparison of our results with some experimental data on self-implanted Si. We also reconsider the migration energetics of single interstitials in the light of recent experiments, and point out the possible role of cluster nucleation in the electrical activation of acceptor-implanted samples

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We describe from advanced first principles calculations the energetics of oxygen doping and its relation to insulator-metal transitions in underdoped YBa2Cu3O6+x. We find a strong tendency of doping oxygens to order into non-magnetic Cu 1+ Ox chains at any x. Ordering produces onedimensional metallic bands, while configurations with non-aligned oxygens are insulating. The Cu 2+ O2 planes remain insulating and antiferromagnetic up to a threshold between x=0.25 and 0.5, above which a paramagnetic normal-metal state prevails. The in-plane antiferro-paramagnetic competition depends on x, but only weakly on the ordering state of the chains.

The normal state of the prototypical high-T c superconductor YBa 2 Cu 3 O 6+y is described via advanced ab initio band-theory techniques suited for strongly correlated materials. We describe the system at generic oxygen doping y between the Mott-insulating limit ͑y =0͒ and the optimally doped Fermi liquid ͑y =1͒, exploring the metal-insulating transitions related to the paramagnetic-antiferromagnetic competition in the CuO 2 planes, and to the order-disorder competition in the CuO y chains. Our study demonstrates the usefulness of ab initio calculations based on density-functional theory, when suitably adapted to strongly correlated systems, to describe doped cuprates.