851 research outputs found
Structure determination of the reconstructed Au(110) surface
The LEED pattern of the Au(110) surface shows a (1 Ă— 2) and also a (1Ă— 3) superstructure. The (1 Ă— 2) superstructure has been determined by comparison of LEED intensities with model calculations. The missing row model is the most probable model. A minimum of the averaged r-factor, , has been found for 15% contraction of the first layer spacing without atomic displacements in the second layer
The structure of K- and Cs-monolayers on Cu(0 0 1): diffraction experiments far from the Bragg point
The intensity analysis along the crystal truncation rods has been used to analyse in situ the adsorption behaviour and the structure of K and Cs on Cu(0 0 1) at submonolayer coverages and room temperature. Up to about 0.25 ML K atoms adsorb in hollow sites followed by formation of a quasihexagonal superstructure. In contrast, for Cs adsorption the data can be interpreted by the formation of quasihexagonal Cs islands that grow with increasing coverage. For K an effective radius of 1.6(1) Ă… independent of coverage is determined. For Cs we fnd d = 2.1 (1) Ă… after formation of the quasihexagonal superstructure
The stability of vicinal surfaces and the equilibrium crystal shape of Pb by first principles theory
The orientation-dependent surface energies of fcc Pb for more than 30 vicinal orientations, distributed over the [110] and [001] zones of the stereographic triangle, have been studied by density-functional theory. For bulk-truncated structures almost all vicinal surfaces are found to be unstable and would facet into (111) and (100) orientations. However, after surface relaxation, all vicinal surfaces are stable relative to faceting into (111) and (100) orientations. There are also regions of relaxed vicinal surfaces which will facet into nearby stable vicinal surfaces. Overall, surface relaxation significantly affects the equilibrium crystal shape (ECS) of Pb. In both the [110] and [001] crystallographic zones the (110), (112), (221), and (023) facets are found on the ECS only after relaxation, in addition to (111) and (100). This result is in agreement with the experimental ECS of Pb at 353 K. Step formation energies for various vicinal orientations are estimated from facet diameters of the theoretical ECS and compared with experimental data
Multilayer distortion in the reconstructed (110) surface of Au
A new LEED intensity analysis of the reconstructed Au(110)-(1Ă—2) surface results in a modification of the missing row model with considerable distortions which are at least three layers deep. The top layer spacing is contracted by about 20%, the second layer exhibits a lateral pairing displacement of 0.07 Ă… and the third layer is buckled by 0.24 Ă…. Distortions in deeper layers seem to be probable but have not been considered in this analysis. The inter-atomic distances in the distorted surface region show both an expansion and a contraction compared to the bulk value and range from 5% contraction to about 4% expansion
Decay of one dimensional surface modulations
The relaxation process of one dimensional surface modulations is re-examined.
Surface evolution is described in terms of a standard step flow model.
Numerical evidence that the surface slope, D(x,t), obeys the scaling ansatz
D(x,t)=alpha(t)F(x) is provided. We use the scaling ansatz to transform the
discrete step model into a continuum model for surface dynamics. The model
consists of differential equations for the functions alpha(t) and F(x). The
solutions of these equations agree with simulation results of the discrete step
model. We identify two types of possible scaling solutions. Solutions of the
first type have facets at the extremum points, while in solutions of the second
type the facets are replaced by cusps. Interactions between steps of opposite
signs determine whether a system is of the first or second type. Finally, we
relate our model to an actual experiment and find good agreement between a
measured AFM snapshot and a solution of our continuum model.Comment: 18 pages, 6 figures in 9 eps file
Profile scaling in decay of nanostructures
The flattening of a crystal cone below its roughening transition is studied
by means of a step flow model. Numerical and analytical analyses show that the
height profile, h(r,t), obeys the scaling scenario dh/dr = F(r t^{-1/4}). The
scaling function is flat at radii r<R(t) \sim t^{1/4}. We find a one parameter
family of solutions for the scaling function, and propose a selection criterion
for the unique solution the system reaches.Comment: 4 pages, RevTex, 3 eps figure
Fluctuations of steps on crystal surfaces
Fluctuations of isolated and pairs of ascending steps of monoatomic height
are studied in the framework of SOS models, using mainly Monte Carlo
techniques. Below the roughening transistion of the surface, the profiles of
long steps show the same scaling features for terrace and surface diffusion.
For a pair of short steps, their separation distance is found to grow as
at late stages. Above roughening, simulational data on surface
diffusion agree well with the classical continuum theory of Mullins.Comment: 4 pages, 2 eps figure
Conception d'un processus amélioré de gestion du risque en développement produit
Le développement de produits est une activité cruciale dans les organisations modernes. Il peut être encadré selon trois domaines orthogonaux : le produit, les activités impliquées dans la création ce dernier, et les personnes responsables de l'exécution de ces activités. Alors que la concurrence se raidit, les organisations sont maintenant non seulement forcées d'introduire continuellement de nouveaux produits, mais aussi de raccourcir leurs délais de développement, de réduire leurs coûts, et d'améliorer la variété et la qualité de leurs produits. Elles ont répondu avec succès à ce défi, en adoptant les pratiques d’ingénierie épurées, en se focalisant sur la valeur créée et en traquant les gaspillages de ressources. Les programmes de qualité et les outils d’amélioration continue permettent également d’accroître la performance résultant du développement. Cependant, la gestion du risque est trop souvent mise à part des outils de planification. Chaque étape du processus, et plus précisément chaque tâche, possède inévitablement des risques. On entend par risque, le déclenchement d’un événement entraînant, pour la tâche, un changement dans le scénario initial. Ce changement de chemin, même si prévu lors d’une planification antérieure, aura forcément un impact sur la performance (délai, coût, qualité) du processus global. Par le biais d'un sondage, effectué auprès d’acteurs de l’industrie aéronautique, cette étude permet de comprendre le niveau de satisfaction et les besoins des utilisateurs du processus de gestion du risque en développement produit. Suite à cela, elle propose un nouveau processus favorisant le recueil et l’exploitation des données de gestion du risque en développement produit
The profile of a decaying crystalline cone
The decay of a crystalline cone below the roughening transition is studied.
We consider local mass transport through surface diffusion, focusing on the two
cases of diffusion limited and attachment-detachment limited step kinetics. In
both cases, we describe the decay kinetics in terms of step flow models.
Numerical simulations of the models indicate that in the attachment-detachment
limited case the system undergoes a step bunching instability if the repulsive
interactions between steps are weak. Such an instability does not occur in the
diffusion limited case. In stable cases the height profile, h(r,t), is flat at
radii r<R(t)\sim t^{1/4}. Outside this flat region the height profile obeys the
scaling scenario \partial h/\partial r = {\cal F}(r t^{-1/4}). A scaling ansatz
for the time-dependent profile of the cone yields analytical values for the
scaling exponents and a differential equation for the scaling function. In the
long time limit this equation provides an exact description of the discrete
step dynamics. It admits a family of solutions and the mechanism responsible
for the selection of a unique scaling function is discussed in detail. Finally
we generalize the model and consider permeable steps by allowing direct adatom
hops between neighboring terraces. We argue that step permeability does not
change the scaling behavior of the system, and its only effect is a
renormalization of some of the parameters.Comment: 25 pages, 18 postscript figure
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