The reconstruction of Rh(001) upon oxygen adsorption

We report on a first-principles study of the structure of O/Rh(001) at half a monolayer of oxygen coverage, performed using density-functional theory. We find that oxygen atoms sit at the center of the black squares of a chess-board, $c(2\times 2)$, pattern. This structure is unstable against a rhomboid distortion of the black squares, which shortens the distance between an O atom and two of the four neighboring Rh atoms, while lengthening the distance with respect to the other two. We actually find that the surface energy is further lowered by allowing the O atom to get off the short diagonal of the rhombus so formed. We predict that the latter distortion is associated with an order-disorder transition, occurring below room temperature. The above rhomboid distortion of the square lattice may be seen as a rotation of the empty, white, squares. Our findings are at variance with recent claims based on STM images, according to which it is instead the black squares which would rotate. We argue that these images are indeed compatible with our predicted reconstruction pattern.Comment: 14 pages (inclusive of 5 figures). To appear on Surface Scienc

Third-order density-functional perturbation theory: a practical implementation with applications to anharmonic couplings in Si

We present a formulation of third-order density-functional perturbation theory which is manifestly invariant with respect to unitary transfomations within the occupied-states manifold and is particularly suitable for a practical implementation of the so called `2n+1' theorem. Our implementation is demonstrated with the calculation of the third-order anharmonic coupling coefficients for some high-simmetry phonons in Silicon.Comment: 6 pages, Plane Tex, SISSA Ref. 78/94/CM/SC (June 94

Floating bonds and gap states in a-Si and a-Si:H from first principles calculations

We study in detail by means of ab-initio pseudopotential calculations the electronic structure of five-fold coordinated (T_5) defects in a-Si and a-Si:H, also during their formation and their evolution upon hydrogenation. The atom-projected densities of states (DOS) and an accurate analysis of the valence charge distribution clearly indicate the fundamental contribution of T_5 defects in originating gap states through their nearest neighbors. The interaction with hydrogen can reduce the DOS in the gap annihilating T_5 defects.Comment: To appear in Europhysics Let

Coordination defects in a-Si and a-Si:H : a characterization from first principles calculations

We study by means of first-principles pseudopotential method the coordination defects in a-Si and a-Si:H, also in their formation and their evolution upon hydrogen interaction. An accurate analysis of the valence charge distribution and of the ``electron localization function'' (ELF) allows to resolve possible ambiguities in the bonding configuration, and in particular to identify clearly three-fold (T_3) and five-fold (T_5) coordinated defects. We found that electronic states in the gap can be associated to both kind of defects, and that in both cases the interaction with hydrogen can reduce the density of states in the gap.Comment: To appear in Philos. Ma

Ab initio study of phonons in wurtzite AlxGa1-xN alloys

We present a theoretical study of the zone-center optical phonons in wurtzite AlxGa1-xN alloys over the whole compositional range from pure GaN to pure AlN. The phonon modes are broadened upon alloying and their frequencies display a blue shift with increasing Al concentration. The E-2 and E-1(TO) modes display a two-mode like behavior and do not preserve a well-defined symmetry in the alloy but rather are mixed and have a large broadening. The LO modes, instead, display a one-mode behavior and have a well-defined symmetry, small broadening, and a pronounced dependence of the frequency upon alloy composition. Therefore, we propose them as the best candidates for the compositional characterization of these materials. (C) 2000 American Institute of Physics. [S0003-6951(00)04815-4]

Interplay between Bonding and Magnetism in the Adsorption of NO on Rh Clusters

We have studied the adsorption of NO on small Rh clusters, containing one to five atoms, using density functional theory in both spin-polarized and non-spin-polarized forms. We find that NO bonds more strongly to Rh clusters than it does to Rh(100) or Rh(111); however, it also quenches the magnetism of the clusters. This (local) effect results in reducing the magnitude of the adsorption energy, and also washes out the clear size-dependent trend observed in the non-magnetic case. Our results illustrate the competition present between the tendencies to bond and to magnetize, in small clusters.Comment: Submitted to J. of Chem. Phy

On-surface and Subsurface Adsorption of Oxygen on Stepped Ag(210) and Ag(410) Surfaces

The adsorption of atomic oxygen and its inclusion into subsurface sites on Ag(210) and Ag(410) surfaces have been investigated using density functional theory. We find that--in the absence of adatoms on the first metal layer--subsurface adsorption results in strong lattice distortion which makes it energetically unfavoured. However subsurface sites are significantly stabilised when a sufficient amount of O adatoms is present on the surface. At high enough O coverage on the Ag(210) surface the mixed on-surface + subsurface O adsorption is energetically favoured with respect to the on-surface only adsorption. Instead, on the Ag(410) surface, at the coverage we have considered (3/8 ML), the existence of stable terrace sites makes the subsurface O incorporation less favourable. These findings are compatible with the results of recent HREEL experiments which have actually motivated this work.Comment: 8 pages, 4 figures and 1 tabl