235 research outputs found
Compendium of meteorological satellites and instrumentation
Pertinent information is presented for 98 launched and planned satellites of the U.S., U.K., U.S.S.R., and France, as well as their over 200 meteorological experiments or instruments. Summary information is provided for both operational and research satellites. Three major sections include: (1) an overview by country, of the various series of meteorological satellite programs; (2) brief descriptions of the satellites and their experiments; and (3) an extensive bibliography. A glossary of acronyms and two indexes for cross-referencing are also included. In addition, various tables and figures presenting satellite operating times, data coverage, location of launch sites, and descriptions of the launch vehicles used to orbit the meteorological satellites are given
Smoluchowski ripening of Ag islands on Ag(100)
Using scanning tunneling microscopy, we study the post-deposition coarsening of distributions of large, two-dimensional Ag islands on a perfect Ag(100) surface at 295 K. The coarsening process is dominated by diffusion, and subsequent collision and coalescence of these islands. To obtain a comprehensive characterization of the coarsening kinetics, we perform tailored families of experiments, systematically varying the initial value of the average island size by adjusting the amount of Ag deposited (up to 0.25 ML). Results unambiguously indicate a strong decrease in island diffusivity with increasing island size. An estimate of the size scaling exponent follows from a mean-field Smoluchowski rate equation analysis of experimental data. These rate equations also predict a rapid depletion in the initial population of smaller islands. This leads to narrowing of the size distribution scaling function from its initial form, which is determined by the process of island nucleation and growth during deposition. However, for later times, a steady increase in the width of this scaling function is predicted, consistent with observed behavior. Finally, we examine the evolution of Ag adlayers on a strained Ag(100) surface, and find significantly enhanced rates for island diffusion and coarsening
Relaxation kinetics in two-dimensional structures
We have studied the approach to equilibrium of islands and pores in two
dimensions. The two-regime scenario observed when islands evolve according to a
set of particular rules, namely relaxation by steps at low temperature and
smooth at high temperature, is generalized to a wide class of kinetic models
and the two kinds of structures. Scaling laws for equilibration times are
analytically derived and confirmed by kinetic Monte Carlo simulations.Comment: 6 pages, 7 figures, 1 tabl
Low temperature shape relaxation of 2-d islands by edge diffusion
We present a precise microscopic description of the limiting step for low
temperature shape relaxation of two dimensional islands in which activated
diffusion of particles along the boundary is the only mechanism of transport
allowed. In particular, we are able to explain why the system is driven
irreversibly towards equilibrium. Based on this description, we present a
scheme for calculating the duration of the limiting step at each stage of the
relaxation process. Finally, we calculate numerically the total relaxation time
as predicted by our results and compare it with simulations of the relaxation
process.Comment: 11 pages, 5 figures, to appear in Phys. Rev.
Changing shapes in the nanoworld
What are the mechanisms leading to the shape relaxation of three dimensional
crystallites ? Kinetic Monte Carlo simulations of fcc clusters show that the
usual theories of equilibration, via atomic surface diffusion driven by
curvature, are verified only at high temperatures. Below the roughening
temperature, the relaxation is much slower, kinetics being governed by the
nucleation of a critical germ on a facet. We show that the energy barrier for
this step linearly increases with the size of the crystallite, leading to an
exponential dependence of the relaxation time.Comment: 4 pages, 5 figures. Accepted by Phys Rev Let
Approaching the low-temperature limit in nucleation and two-dimensional growth of fcc (100) metal films Ag/Ag(100)
We analyze the formation of two-dimensional Ag islands following deposition of about 0.1 ML of Ag on Ag(100) over a temperature regime ranging from classical nucleation and growth behavior to almost immobile adatoms, from 300 to 125 K. Particular emphasis is placed on the post-deposition dynamics at the lower end of the temperature range, where the saturation island density is not reached at the end of the deposition, and nucleation and aggregation processes continue with adatoms from the remaining adatom gas. Our analysis combines VT scanning tunneling microscopy experiments with kinetic Monte Carlo simulation of appropriate atomistic models. The only adjustable parameters in the model are the terrace diffusion barrier and prefactor, which can be determined from island density behavior near room temperature. Other processes such as rapid edge diffusion, and “easy” nucleation and aggregation of diagonally adjacent adatoms, are treated as instantaneous. The model excellently reproduces all aspects of behavior at low temperatures, demonstrating that nucleation and growth processes can be described in one consistent scheme, down to the regime of almost immobile adatoms
Formation and Equilibration of Submonolayer Island Distributions in Ag/Ag(100) Homoepitaxy
We present an analysis both of the nucleation and growth of two-dimensional (2D) islands or clusters during deposition of Ag on Ag(100) at 295 K and of the subsequent postdeposition equilibration of such island distributions at coverages below about 0.25 monolayer. Island formation during deposition is shown to be effectively irreversible, and the island density and size and separation distributions are characterized using a combination of scanning tunneling microscopy (STM) and high-resolution low-energy electron diffraction. Postdeposition coarsening of the adlayer is monitored via STM and is shown to be dominated typically by diffusion and subsequent coalescence of large 2D clusters rather than by Ostwald ripening. Tailored studies of such coarsening elucidate both the kinetics and the underlying cluster diffusion process
Instability driven fragmentation of nanoscale fractal islands
Formation and evolution of fragmentation instabilities in fractal islands,
obtained by deposition of silver clusters on graphite, are studied. The
fragmentation dynamics and subsequent relaxation to the equilibrium shapes are
controlled by the deposition conditions and cluster composition. Sharing common
features with other materials' breakup phenomena, the fragmentation instability
is governed by the length-to-width ratio of the fractal arms.Comment: 5 pages, 3 figures, Physical Review Letters in pres
Electromigration of Single-Layer Clusters
Single-layer atom or vacancy clusters in the presence of electromigration are
studied theoretically assuming an isotropic medium. A variety of distinctive
behaviors distinguish the response in the three standard limiting cases of
periphery diffusion (PD), terrace diffusion (TD), and evaporation-condensation
(EC). A general model provides power laws describing the size dependence of the
drift velocity in these limits, consistent with established results in the case
of PD. The validity of the widely used quasistatic limit is calculated. Atom
and vacancy clusters drift in opposite directions in the PD limit but in the
same direction otherwise. In absence of PD, linear stability analysis reveals a
new type of morphological instability, not leading to island break-down. For
strong electromigration, Monte Carlo simulations show that clusters then
destabilize into slits, in contrast to splitting in the PD limit.
Electromigration affects the diffusion coefficient of the cluster and
morphological fluctuations, the latter diverging at the instability threshold.
An instrinsic attachment-detachment bias displays the same scaling signature as
PD in the drift velocity.Comment: 11 pages, 4 figure
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