71 research outputs found
Fast coarsening in unstable epitaxy with desorption
Homoepitaxial growth is unstable towards the formation of pyramidal mounds
when interlayer transport is reduced due to activation barriers to hopping at
step edges. Simulations of a lattice model and a continuum equation show that a
small amount of desorption dramatically speeds up the coarsening of the mound
array, leading to coarsening exponents between 1/3 and 1/2. The underlying
mechanism is the faster growth of larger mounds due to their lower evaporation
rate.Comment: 4 pages, 4 PostScript figure
Numerical test of the damping time of layer-by-layer growth on stochastic models
We perform Monte Carlo simulations on stochastic models such as the
Wolf-Villain (WV) model and the Family model in a modified version to measure
mean separation between islands in submonolayer regime and damping time
of layer-by-layer growth oscillations on one dimension. The
stochastic models are modified, allowing diffusion within interval upon
deposited. It is found numerically that the mean separation and the damping
time depend on the diffusion interval , leading to that the damping time is
related to the mean separation as for the WV model
and for the Family model. The numerical results are in
excellent agreement with recent theoretical predictions.Comment: 4 pages, source LaTeX file and 5 PS figure
Coarsening Dynamics of Crystalline Thin Films
The formation of pyramid-like structures in thin-film growth on substrates
with a quadratic symmetry, e.g., {001} surfaces, is shown to exhibit
anisotropic scaling as there exist two length scales with different time
dependences. Analytical and numerical results indicate that for most
realizations coarsening of mounds is described by an exponent n=0.2357.
However, depending on material parameters, n may lie between 0 (logarithmic
coarsening) and 1/3. In contrast, growth on substrates with triangular
symmetries ({111} surfaces) is dominated by a single length scale and an
exponent n=1/3.Comment: RevTeX, 4 pages, 3 figure
Dissociation of O2 molecules on strained Pb(111) surfaces
By performing first-principles molecular dynamics calculations, we
systematically simulate the adsorption behavior of oxygen molecules on the
clean and strained Pb(111) surfaces. The obtained molecular adsorption
precursor state, and the activated dissociation process for oxygen molecules on
the clean Pb surface are in good agreements with our previous static
calculations, and perfectly explains previous experimental observations [Proc.
Natl. Acad. Sci. U.S.A. 104, 9204 (2007)]. In addition, we also study the
influences of surface strain on the dissociation behaviors of O2 molecules. It
is found that on the compressed Pb(111) surfaces with a strain value of larger
than 0.02, O2 molecules will not dissociate at all. And on the stretched
Pb(111) surfaces, O2 molecules become easier to approach, and the adsorption
energy of the dissociated oxygen atoms is larger than that on the clean Pb
surface
How metal films de-wet substrates - identifying the kinetic pathways and energetic driving forces
We study how single-crystal chromium films of uniform thickness on W(110)
substrates are converted to arrays of three-dimensional (3D) Cr islands during
annealing. We use low-energy electron microscopy (LEEM) to directly observe a
kinetic pathway that produces trenches that expose the wetting layer. Adjacent
film steps move simultaneously uphill and downhill relative to the staircase of
atomic steps on the substrate. This step motion thickens the film regions where
steps advance. Where film steps retract, the film thins, eventually exposing
the stable wetting layer. Since our analysis shows that thick Cr films have a
lattice constant close to bulk Cr, we propose that surface and interface stress
provide a possible driving force for the observed morphological instability.
Atomistic simulations and analytic elastic models show that surface and
interface stress can cause a dependence of film energy on thickness that leads
to an instability to simultaneous thinning and thickening. We observe that
de-wetting is also initiated at bunches of substrate steps in two other
systems, Ag/W(110) and Ag/Ru(0001). We additionally describe how Cr films are
converted into patterns of unidirectional stripes as the trenches that expose
the wetting layer lengthen along the W[001] direction. Finally, we observe how
3D Cr islands form directly during film growth at elevated temperature. The Cr
mesas (wedges) form as Cr film steps advance down the staircase of substrate
steps, another example of the critical role that substrate steps play in 3D
island formation
Factors influencing graphene growth on metal surfaces
Graphene forms from a relatively dense, tightly-bound C-adatom gas, when
elemental C is deposited on or segregates to the Ru(0001) surface. Nonlinearity
of the graphene growth rate with C adatom density suggests that growth proceeds
by addition of C atom clusters to the graphene edge. The generality of this
picture has now been studied by use of low-energy electron microscopy (LEEM) to
observe graphene formation when Ru(0001) and Ir(111) surfaces are exposed to
ethylene. The finding that graphene growth velocities and nucleation rates on
Ru have precisely the same dependence on adatom concentration as for elemental
C deposition implies that hydrocarbon decomposition only affects graphene
growth through the rate of adatom formation; for ethylene, that rate decreases
with increasing adatom concentration and graphene coverage. Initially, graphene
growth on Ir(111) is like that on Ru: the growth velocity is the same nonlinear
function of adatom concentration (albeit with much smaller equilibrium adatom
concentrations, as we explain with DFT calculations of adatom formation
energies). In the later stages of growth, graphene crystals that are rotated
relative to the initial nuclei nucleate and grow. The rotated nuclei grow much
faster. This difference suggests first, that the edge-orientation of the
graphene sheets relative to the substrate plays an important role in the growth
mechanism, and second, that attachment of the clusters to the graphene is the
slowest step in cluster addition, rather than formation of clusters on the
terraces
Host suitability of weeds and forage species to root-knot nematode meloidogyne graminicola as a funcion of irrigation management
Deformations Preserving Gauß Curvature
(Proceedings of LHMTS 2013)International audienceIn industrial surface generation, it is important to consider surfaces with minimal areas for two main reasons: these surfaces require less material than non-minimal surfaces, and they are cheaper to manufacture. Based on a prototype, a so-called masterpiece, the final product is created using small deformations to adapt a surface to the desired shape. We present a linear deformation technique preserving the total curvature of the masterpiece. In particular, we derive sufficient conditions for these linear deformations to be total curvature preserving when applied to the masterpiece. It is useful to preserve total curvature of a surface in order to minimise the amount of material needed, and to minimise bending energy
Euclid: modelling massive neutrinos in cosmology - a code comparison
Material outgassing in a vacuum leads to molecular contamination, a well-known problem in spaceflight. Water is the most common contaminant in cryogenic spacecraft, altering numerous properties of optical systems. Too much ice means that Euclid’s calibration requirements cannot be met anymore. Euclid must then be thermally decontaminated, which is a month-long risky operation. We need to understand how ice affects our data to build adequate calibration and survey plans. A comprehensive analysis in the context of an astrophysical space survey has not been done before. In this paper we look at other spacecraft with well-documented outgassing records. We then review the formation of thin ice films, and find that for Euclid a mix of amorphous and crystalline ices is expected. Their surface topography – and thus optical properties – depend on the competing energetic needs of the substrate-water and the water-water interfaces, and they are hard to predict with current theories. We illustrate that with scanning-tunnelling and atomic-force microscope images of thin ice films. Sophisticated tools exist to compute contamination rates, and we must understand their underlying physical principles and uncertainties. We find considerable knowledge errors on the diffusion and sublimation coefficients, limiting the accuracy of outgassing estimates. We developed a water transport model to compute contamination rates in Euclid, and find agreement with industry estimates within the uncertainties. Tests of the Euclid flight hardware in space simulators did not pick up significant contamination signals, but they were also not geared towards this purpose; our in-flight calibration observations will be much more sensitive. To derive a calibration and decontamination strategy, we need to understand the link between the amount of ice in the optics and its effect on the data. There is little research about this, possibly because other spacecraft can decontaminate more easily, quenching the need for a deeper understanding. In our second paper, we quantify the impact of iced optics on Euclid’s data
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