747 research outputs found
Fluctuations, line tensions, and correlation times of nanoscale islands on surfaces
We analyze in detail the fluctuations and correlations of the (spatial)
Fourier modes of nano-scale single-layer islands on (111) fcc crystal surfaces.
We analytically show that the Fourier modes of the fluctuations couple due to
the anisotropy of the crystal, changing the power spectrum of the fluctuations,
and that the actual eigenmodes of the fluctuations are the appropriate linear
combinations of the Fourier modes. Using kinetic Monte Carlo simulations with
bond-counting parameters that best match realistic energy barriers for hopping
rates, we deduce absolute line tensions as a function of azimuthal orientation
from the analyses of the fluctuation of each individual mode. The
autocorrelation functions of these modes give the scaling of the correlation
times with wavelength, providing us with the rate-limiting kinetics driving the
fluctuations, here step-edge diffusion. The results for the energetic
parameters are in reasonable agreement with available experimental data for
Pb(111) surfaces, and we compare the correlation times of island-edge
fluctuations to relaxation times of quenched Pb crystallites.Comment: 11 pages, 8 figures; to appear in PRB 70, xxx (15 Dec 2004), changes
in MC and its implication
Anomalous Dimension and Spatial Correlations in a Point-Island Model
We examine the island size distribution function and spatial correlation
function of a model for island growth in the submonolayer regime in both 1 and
2 dimensions. In our model the islands do not grow in shape, and a fixed number
of adatoms are added, nucleate, and are trapped at islands as they diffuse.
We study the cases of various critical island sizes for nucleation as a
function of initial coverage. We found anomalous scaling of the island size
distribution for large . Using scaling, random walk theory, a version of
mean-field theory we obtain a closed form for the spatial correlation function.
Our analytic results are verified by Monte Carlo simulations
A lattice gas model of II-VI(001) semiconductor surfaces
We introduce an anisotropic two-dimensional lattice gas model of metal
terminated II-IV(001) seminconductor surfaces. Important properties of this
class of materials are represented by effective NN and NNN interactions, which
result in the competition of two vacancy structures on the surface. We
demonstrate that the experimentally observed c(2x2)-(2x1) transition of the
CdTe(001) surface can be understood as a phase transition in thermal
equilbrium. The model is studied by means of transfer matrix and Monte Carlo
techniques. The analysis shows that the small energy difference of the
competing reconstructions determines to a large extent the nature of the
different phases. Possible implications for further experimental research are
discussed.Comment: 7 pages, 2 figure
Annihilation of Immobile Reactants on the Bethe Lattice
Two-particle annihilation reaction, A+A -> inert, for immobile reactants on
the Bethe lattice is solved exactly for the initially random distribution. The
process reaches an absorbing state in which no nearest-neighbor reactants are
left. The approach of the concentration to the limiting value is exponential.
The solution reproduces the known one-dimensional result which is further
extended to the reaction A+B -> inert.Comment: 12 pp, TeX (plain
Effects of Impurities in Random Sequential Adsorption on a One-Dimensional Substrate
We have solved the kinetics of random sequential adsorption of linear
-mers on a one-dimensional disordered substrate for the random sequential
adsorption initial condition and for the random initial condition. The jamming
limits at fixed length of linear -mers have a
minimum point at a particular density of the linear -mers impurity for both
cases. The coverage of the surface and the jamming limits are compared to the
results for Monte Carlo simulation. The Monte Carlo results for the jamming
limits are in good agreement with the analytical results. The continuum limits
are derived from the analytical results on lattice substrates.Comment: 9 pages, latex, 1 figure not included, accepted in Phys. Rev.
Scaling of Island Growth in Pb Overlayers on Cu(001)
The growth and ordering of a Pb layer deposited on Cu(001) at 150 K has been
studied using atom beam scattering. At low coverage, ordered Pb islands with a
large square unit cell and nearly hexagonal internal structure are formed. This
is a high order commensurate phase with 30 atoms in the unit cell. From the
measurement of the island diffraction peak profiles we find a power law for the
mean island - size versus coverage with an exponent . A
scaling behavior of growth is confirmed and a simple model describing island
growth is presented. Due to the high degeneracy of the monolayer phase,
different islands do not diffract coherently. Therefore, when islands merge
they still diffract as separate islands and coalescence effects are thus
negligible. From the result for we conclude that the island density is
approximately a constant in the coverage range where the
ordered islands are observed. We thus conclude that most islands nucleate at
and then grow in an approximately self similar fashion as
increases.Comment: 23 pages, 10 Figures (available upon request). SU-PHYS-93-443-375
The cytoskeletal system of nucleated erythrocytes. I. Composition and function of major elements.
ABSTRACT We have studied the dogfish erythrocyte cytoskeletal system, which consists of a marginal band of microtubules (MB) and trans-marginal band material (TBM). The TBM appeared in whole mounts as a rough irregular network and in thin sections as a surfacedelimiting layer completely enclosing nucleus and MB. In cells incubated at 0°C for 30 min or more, the MB disappeared but the TBM remained. MB reassembly occurred with rewarming, and was inhibited by colchicine. Flattened elliptical erythrocyte morphology was retained even when MBs were absent. Total solubilization of MB and TBM at low pH, or dissolution of whole anucleate cytoskeletons, yielded components comigrating with actin, spectrin, and tubulin standards during gel electrophoresis. Mass-isolated MBs, exhibiting ribbonlike construction apparently maintained by cross-bridges, contained four polypeptides in the tubulin region of the gel. Only these four bands were noticeably increased in the soluble phase obtained from cells with 0°C-disassembled MBs. The best isolated MB preparations contained tubulin but no components comigrating with high molecular weight microtubule-associated proteins, spectrin, or actin. Actin and spectrin therefore appear to be major TBM constituents, with tubulin localized in the MB. The results are interpreted in terms of an actin- and spectrin-containing subsurface cytoskeletal layer (TBM), related to that of mammalian erythrocytes, which maintains cell shape in the absence of MBs. Observations on abnormal pointed erythrocytes containing similarly pointed MBs indicate further that the MB can deform the TBM from within so as to alter cell shape. MBs may function in this manner during normal cellular morphogenesis and during blood flow in vivo. Marginal bands of microtubules (MBs) are a major structural festure of all nonmammalian vertebrate erythrocytes and variou
A quantitative theory of current-induced step bunching on Si(111)
We use a one-dimensional step model to study quantitatively the growth of
step bunches on Si(111) surfaces induced by a direct heating current.
Parameters in the model are fixed from experimental measurements near 900 deg C
under the assumption that there is local mass transport through surface
diffusion and that step motion is limited by the attachment rate of adatoms to
step edges. The direct heating current is treated as an external driving force
acting on each adatom. Numerical calculations show both qualitative and
quantitative agreement with experiment. A force in the step down direction will
destabilize the uniform step train towards step bunching. The average size of
the step bunches grows with electromigration time as t^beta, with beta = 0.5,
in agreement with experiment and with an analytical treatment of the steady
states. The model is extended to include the effect of direct hopping of
adatoms between different terraces. Monte-Carlo simulations of a solid-on-solid
model, using physically motivated assumptions about the dynamics of surface
diffusion and attachment at step edges, are carried out to study two
dimensional features that are left out of the present step model and to test
its validity. These simulations give much better agreement with experiment than
previous work. We find a new step bending instability when the driving force is
along the step edge direction. This instability causes the formation of step
bunches and antisteps that is similar to that observed in experiment.Comment: 11 pages, 7 figure
A contiuum model for low temperature relaxation of crystal steps
High and low temperature relaxation of crystal steps are described in a
unified picture, using a continuum model based on a modified expression of the
step free energy. Results are in agreement with experiments and Monte Carlo
simulations of step fluctuations and monolayer cluster diffusion and
relaxation. In an extended model where mass exchange with neighboring terraces
is allowed, step transparency and a low temperature regime for unstable step
meandering are found.Comment: Submitted to Phys.Rev.Let
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
- …