Materials Theory

Electron-hole excitations in the surface bands of GaAs(110) are analyzed using constrained density-functional theory calculations. The results show that Frenkel-type autolocalized excitons are formed. The excitons induce a local surface unrelaxation which results in a strong exciton-exciton attraction and makes complexes of two or three electron-hole pairs more favorable than separate excitons. In such microscopic exciton "droplets" the electron density is mainly concentrated in the dangling orbital of a surface Ga atom whereas the holes are distributed over the bonds of this atom to its As neighbors thus weakening the bonding to the substrate. This finding suggests the microscopic mechanism of a laser-induced emission of neutral Ga atoms from GaAs and GaP (110) surfaces.

We present an {\it ab initio} approach for the computation of the magnetic susceptibility χ of insulators. The approach is applied to compute χ in diamond and in solid neon using density functional theory in the local density approximation, obtaining good agreement with experimental data. In solid neon, we predict an observable dependence of χ upon pressure.

Fused Silica is the material of choice for UV optical systems such as the projection optics in microlithography systems. Unfortunately fused silica is not stable under UV irradition and undergoes compaction and color center formation. The UV damage properties of fused silica have been the subject of a number of studies. The detailed microscopic mechanism for color center formation is complex and remains obscure but the fact that compaction must be a generic property of a glassy system seems to have been overlooked by most researchers although Krajnovich, et. al., do briefly discuss compaction with respect to a specific glass model. The purpose of this note is to briefly discuss how the basic mechanism of compaction follows directly from the modern theoretical understanding of glassy systems. This understanding of the basic mechanism of compaction fits the experimental data quite well. It also allows for the prediction of some of qualitative properties of compaction which have not so far been experimentally determined, such as, the absence of a minimum damage threshold for the onset of compaction and the eventual long term cessation of compaction after sufficient shrinkage has occurred.

The modification of the potential-energy surface (PES) of H_2 dissociation over Pd(100) as induced by the presence of a (2x2) S adlayer is investigated by density-functional theory and the linear augmented plane wave method. It is shown that the poisoning effect of S originates from the formation of energy barriers hindering the dissociation of H_2. The barriers are in the entrance channel of the PES and their magnitude strongly depends on the lateral distance of the H_2 molecule from the S adatoms.

A metastable phase of NbN with superconducting Tc=16.4 K was reported recently by Treece and collaborators. The reported structure of the thin film sample deviates from the rocksalt (B1) NbN structure with 25% ordered vacancies on each sublattice (space group Pm3m) and a lattice constant of 4.442 Angstroms. Using full potential electronic structure methods, we contrast the electronic structure with that of B1 NbN. The calculated energy, 1.00 eV/molecule higher than B1 NbN, and calculated lattice constant of 4.214 Angstroms indicate that the new phase must be something other than the ordered stoichiometric Pm3m phase.

We explore the effect of faceting on possible mechanisms for mass transport around electromigration voids in aluminum interconnects. Motivated by linear response estimates which suggest that particle flux would be much higher along steps than across terraces on a clean aluminum surface, we study step nucleation in the presence of a small driving force along a surface. We find that step nucleation, even on a nearly defect-free void surface, would be slow if the step energy is equal to that calculated for a clean aluminum surface. In the presence of a uniform electromigration force, the creation of new steps between existing ones should not occur unless the free energy cost of a step is much less than thermal energies. We conclude that voids cannot move intragranularly at μ m/hr rates without help from other factors such as local heating and impurities.

We provide a systematic test of empirical theories of covalent bonding in solids using an exact procedure to invert ab initio cohesive energy curves. By considering multiple structures of the same material, it is possible for the first time to test competing angular functions, expose inconsistencies in the basic assumption of a cluster expansion, and extract general features of covalent bonding. We test our methods on silicon, and provide the direct evidence that the Tersoff-type bond order formalism correctly describes coordination dependence. For bond-bending forces, we obtain skewed angular functions that favor small angles, unlike existing models. As a proof-of-principle demonstration, we derive a Si interatomic potential which exhibits comparable accuracy to existing models.

A phenomenological model of the evolution of an ensemble of interacting dislocations in an isotropic elastic medium is formulated. The line-defect microstructure is described in terms of a spatially coarse-grained order parameter, the dislocation density tensor. The tensor field satisfies a conservation law that derives from the conservation of Burgers vector. Dislocation motion is entirely dissipative and is assumed to be locally driven by the minimization of plastic free energy. We first outline the method and resulting equations of motion to linear order in the dislocation density tensor, obtain various stationary solutions, and give their geometric interpretation. The coupling of the dislocation density to an externally imposed stress field is also addressed, as well as the impact of the field on the stationary solutions.

Energy storage and release in dielectric materials can be described on the basis of the charge trapping mechanism. Most phenomenological aspects have been recently rationalized in terms of the space charge model~\cite{blaise,blaise1}. Dynamical aspects are studied here by performing Molecular Dynamics simulations. We show that an excess electron introduced into the sapphire lattice (\alumina) can be trapped only at a limited number of sites. The energy gained by allowing the electron to localize in these sites is of the order of 4-5 eV, in good agreement with the results of the space charge model. Displacements of the neighboring ions due to the implanted charge are shown to be localized in a small region of about 5~Å. Detrapping is observed at 250 K . The ionic displacements turn out to play an important role in modifying the potential landscape by lowering, in a dynamical way, the barriers that cause localization at low temperature.

We present a method for reliably determining the lowest energy structure of an atomic cluster in an arbitrary model potential. The method is based on a genetic algorithm, which operates on a population of candidate structures to produce new candidates with lower energies. Our method dramatically outperforms simulated annealing, which we demonstrate by applying the genetic algorithm to a tight-binding model potential for carbon. With this potential, the algorithm efficiently finds fullerene cluster structures up to C 60 starting from random atomic coordinates.