354 research outputs found
Capture-zone scaling in island nucleation: phenomenological theory of an example of universal fluctuation behavior
In studies of island nucleation and growth, the distribution of capture
zones, essentially proximity cells, can give more insight than island-size
distributions. In contrast to the complicated expressions, ad hoc or derived
from rate equations, usually used, we find the capture-zone distribution can be
described by a simple expression generalizing the Wigner surmise from random
matrix theory that accounts for the distribution of spacings in a host of
fluctuation phenomena. Furthermore, its single adjustable parameter can be
simply related to the critical nucleus of growth models and the substrate
dimensionality. We compare with extensive published kinetic Monte Carlo data
and limited experimental data. A phenomenological theory sheds light on the
result.Comment: 5 pages, 4 figures, originally submitted to Phys. Rev. Lett. on Dec.
15, 2006; revised version v2 tightens and focuses the presentation,
emphasizes the importance of universal features of fluctuations, corrects an
error for d=1, replaces 2 of the figure
Crossover in the scaling of island size and capture zone distributions
Simulations of irreversible growth of extended (fractal and square) islands
with critical island sizes i=1 and 2 are performed in broad ranges of coverage
\theta and diffusion-to-deposition ratios R in order to investigate scaling of
island size and capture zone area distributions (ISD, CZD). Large \theta and
small R lead to a crossover from the CZD predicted by the theory of Pimpinelli
and Einstein (PE), with Gaussian right tail, to CZD with simple exponential
decays. The corresponding ISD also cross over from Gaussian or faster decays to
simple exponential ones. For fractal islands, these features are explained by
changes in the island growth kinetics, from a competition for capture of
diffusing adatoms (PE scaling) to aggregation of adatoms with effectively
irrelevant diffusion, which is characteristic of random sequential adsorption
(RSA) without surface diffusion. This interpretation is confirmed by studying
the crossover with similar CZ areas (of order 100 sites) in a model with
freezing of diffusing adatoms that corresponds to i=0. For square islands,
deviations from PE predictions appear for coverages near \theta=0.2 and are
mainly related to island coalescence. Our results show that the range of
applicability of the PE theory is narrow, thus observing the predicted Gaussian
tail of CZD may be difficult in real systems.Comment: 9 pages, 7 figure
Labyrinthine Island Growth during Pd/Ru(0001) Heteroepitaxy
Using low energy electron microscopy we observe that Pd deposited on Ru only
attaches to small sections of the atomic step edges surrounding Pd islands.
This causes a novel epitaxial growth mode in which islands advance in a
snakelike motion, giving rise to labyrinthine patterns. Based on density
functional theory together with scanning tunneling microscopy and low energy
electron microscopy we propose that this growth mode is caused by a surface
alloy forming around growing islands. This alloy gradually reduces step
attachment rates, resulting in an instability that favors adatom attachment at
fast advancing step sections
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
Unequal access to testing and vaccination services for the homeless and undocumented population during the COVID-19 pandemic
Objectives: To furnish a model to ensure access and use of healthcare services to the undocumented and homeless population. Methods: Between March 2020 and October 2021, public and third sector actors in Rome implemented an accessible COVID-19 screening service and vaccination program targeting the homeless and undocumented population. Results: 95.6% of the patients tested negative to both rapid and molecular tests. 0.9% tested positive to both. 0.7% were false negatives, while 2.8% were false positives. None of the participants refused the diagnostic treatment. From July to October 2021, 1384 people received a complete cycle of the COVID-19 vaccine through the program. 632 (45.6%) also agreed to perform the antibodies testing before inoculation. 318 (50.31%) of these were positive at the time of vaccination. Conclusion: We present a cost-effective model for reducing structural barriers to access diagnostic and preventive services for the homeless and undocumented population that can be applied to different public health settings
Dendritic cells: ultrastructural and immunophenotypical changes upon nb-UVB in vitiligo skin.
Scaling properties of step bunches induced by sublimation and related mechanisms: A unified perspective
This work provides a ground for a quantitative interpretation of experiments
on step bunching during sublimation of crystals with a pronounced
Ehrlich-Schwoebel (ES) barrier in the regime of weak desorption. A strong step
bunching instability takes place when the kinetic length is larger than the
average distance between the steps on the vicinal surface. In the opposite
limit the instability is weak and step bunching can occur only when the
magnitude of step-step repulsion is small. The central result are power law
relations of the between the width, the height, and the minimum interstep
distance of a bunch. These relations are obtained from a continuum evolution
equation for the surface profile, which is derived from the discrete step
dynamical equations for. The analysis of the continuum equation reveals the
existence of two types of stationary bunch profiles with different scaling
properties. Through a mathematical equivalence on the level of the discrete
step equations as well as on the continuum level, our results carry over to the
problems of step bunching induced by growth with a strong inverse ES effect,
and by electromigration in the attachment/detachment limited regime. Thus our
work provides support for the existence of universality classes of step
bunching instabilities [A. Pimpinelli et al., Phys. Rev. Lett. 88, 206103
(2002)], but some aspects of the universality scenario need to be revised.Comment: 21 pages, 8 figure
SARS-CoV-2 infection prevalence in people experiencing homelessness
Objective: People experiencing homelessness have peculiar characteristics that make them more vulnerable to Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) transmission and to more serious forms of Coronavirus Disease 19 (COVID-19). The aim of this study was to evaluate the prevalence of SARS-CoV-2 infection in the homeless population assisted by the primary care services of the Eleemosynaria Apostolica, Vatican City. Patients and methods: Persons experiencing homelessness and the volunteers assisting them were tested for COVID-19 through PCR and antigen rapid test between October 1st, 2020, and June 5th, 2021, in the clinical facilities of the Eleemosynaria Apostolica. Results: A total of 1665 subjects from 96 different countries in five continents were included in the study; age range was 1-90 years. Overall, 2315 COVID-19 tests through nasopharyngeal swab were performed; 1052 Polymerase Chain Reaction (PCR) tests and 1263 antigen rapid tests. Nearly 40% of the subjects underwent both tests (n=650, 39.04%), 402 were tested with PCR test only (24.14%) and 613 with antigen test only (36.8%). PCR tests were negative in 966 cases and positive in 86 (8.17%), while antigen tests were negative in 1205 cases and positive in 58 (4.59%). The number of positive cases varied over time, with a drastic increase during the winter months of 2020 and a progressive decrease over 2021. Among positive cases, 24.41% were symptomatic; symptoms included fever, breathing difficulties, anosmia/hyposmia, cough, headache, and diarrhea. Conclusions: This study reported an overall prevalence of SARS-CoV-2 infection in our sample slightly above 8%. Additional data on viral genome through sequencing of SARS-CoV-2 in positive cases are of utmost importance to help identify variants and implement specific infection control measures
Characterization of functional opioid delta receptors in a luteinizing hormone-releasing hormone-producing neuronal cell line
Endogenous opioids participate in the regulation of gonadotropin secretion through an influence on the release of the hypothalamic LHRH. However, it is not clear whether opioids exert a direct effect on LHRH-producing neurons or interfere with other systems able to influence LHRH release. A neuronal LHRH-producing cell line (GT1) developed recently provides a good model to study the mechanisms controlling LHRH release. In the present study, the presence of opioid receptors on a subclone of GT1 cells (GT1-1) has been investigated. A specific and saturable binding of the 3H-labeled nonselective opioid ligand diprenorphine ([3H]DIP) was detected by a receptor binding assay on both intact GT1-1 cells and crude membrane preparations obtained from these cells. Analysis of saturation curves revealed that [3H]DIP apparently binds to a single class of sites with a Kd of 0.2 nM and a binding capacity of 125 fmol/mg protein, corresponding to approximately 20,000 sites/cell. Selective displacement of the binding of [3H]DIP to GT1-1 cells by [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin, [D-Pen2,D-Pen5]enkephalin (DPDPE), and U50488H, which are selective ligands, respectively, for mu-, delta-, and kappa-receptors, was also evaluated. Only the specific delta-ligand DPDPE produced a significant inhibition of the binding of [3H]DIP. [D-Ala2,N-Me-Phe4,Gly5-ol]Enkephalin and U50488H were totally ineffective. The inhibitory effect of the agonist DPDPE on the binding of [3H]DIP was decreased by the presence of sodium ions, a typical characteristic of the binding of agonists to opioid receptors. Finally, it has been observed that treatment with prostaglandins E1 and E2 produces a dramatic increase in cAMP accumulation in GT1-1 cells, and DPDPE is highly effective in suppressing this effect. On the basis of these results, it is possible to postulate the presence of functional delta-opioid receptors on GT1-1 cells. By extrapolation, one might suggest that endogenous opioids may affect LHRH neurons by two mechanisms: a direct one, acting via delta-receptors, and an indirect one, through the activation of neurons impinging on the LHRH system, which uses mu-receptors
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
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