514 research outputs found
Random sequential adsorption on a dashed line
We study analytically and numerically a model of random sequential adsorption
(RSA) of segments on a line, subject to some constraints suggested by two kinds
of physical situations:
- deposition of dimers on a lattice where the sites have a spatial extension;
- deposition of extended particles which must overlap one (or several)
adsorbing sites on the substrate.
Both systems involve discrete and continuous degrees of freedom, and, in one
dimension, are equivalent to our model, which depends on one length parameter.
When this parameter is varied, the model interpolates between a variety of
known situations : monomers on a lattice, "car-parking" problem, dimers on a
lattice. An analysis of the long-time behaviour of the coverage as a function
of the parameter exhibits an anomalous 1/t^2 approach to the jamming limit at
the transition point between the fast exponential kinetics, characteristic of
the lattice model, and the 1/t law of the continuous one.Comment: 14 pages (Latex) + 4 Postscript figure
Adsorption of Line Segments on a Square Lattice
We study the deposition of line segments on a two-dimensional square lattice.
The estimates for the coverage at jamming obtained by Monte-Carlo simulations
and by -order time-series expansion are successfully compared. The
non-trivial limit of adsorption of infinitely long segments is studied, and the
lattice coverage is consistently obtained using these two approaches.Comment: 19 pages in Latex+5 postscript files sent upon request ; PTB93_
Critical properties of Ising model on Sierpinski fractals. A finite size scaling analysis approach
The present paper focuses on the order-disorder transition of an Ising model
on a self-similar lattice. We present a detailed numerical study, based on the
Monte Carlo method in conjunction with the finite size scaling method, of the
critical properties of the Ising model on some two dimensional deterministic
fractal lattices with different Hausdorff dimensions. Those with finite
ramification order do not display ordered phases at any finite temperature,
whereas the lattices with infinite connectivity show genuine critical behavior.
In particular we considered two Sierpinski carpets constructed using different
generators and characterized by Hausdorff dimensions d_H=log 8/log 3 = 1.8927..
and d_H=log 12/log 4 = 1.7924.., respectively.
The data show in a clear way the existence of an order-disorder transition at
finite temperature in both Sierpinski carpets.
By performing several Monte Carlo simulations at different temperatures and
on lattices of increasing size in conjunction with a finite size scaling
analysis, we were able to determine numerically the critical exponents in each
case and to provide an estimate of their errors.
Finally we considered the hyperscaling relation and found indications that it
holds, if one assumes that the relevant dimension in this case is the Hausdorff
dimension of the lattice.Comment: 21 pages, 7 figures; a new section has been added with results for a
second fractal; there are other minor change
Perspectives on Andean Prehistory and Protohistory: Papers from the Third Annual Northeast Conference on Andean Archaeology and Ethnohistory
This volume represents eight of the eighteen papers presented at the Third Northeast Conference on Andean Archaeology and Ethnohistory held at the University of Massachusetts, Amherst on October 27 and 28, 1984. It also includes a paper presented at the Second NCAAE held at the American Museum of Natural History on November 19-20, 1983. The papers include: Wandering Shellfish: New Insights from Southeastern Coastal Ecuador by Patricia Netherly, Late Prehistoric Terracing at Chijra in the Collca Valley, Peru: Preliminary Report I by Michael A. Malpass, The Topara Tradition: An Overview by Dwight T. Wallace, The Peruvian North Central Coast During the Early Intermediate Period: An Emerging Perspective by Richard E. Daggett, A Sequence of Monumental Architecture from Huamanchuco by John R. Topic, Duality in Public Architecture in the Upper Zena Valley by Patricia J. Netherly and Tom D. Dillehay, Piruru: A Preliminary Report on the Archaeological Botany of a Highland Andean Site by Lawrence Kaplan and Elisabeth Bonnier, Analysis of Organic Remains from Huamachuco Qollqas by Coreen E. Chiswell, Aspects of Casting Practice in Prehispanic Peru by Stuart V. Arnold, and Representations of the Cosmos: A Comparison of the Church of San Cristobal de Pampachiri with the Coricancha Drawing of Santacruz Pachacuti Yamqui Salcamaygua by Monica Barnes.https://digitalcommons.library.umaine.edu/andean_past_special/1000/thumbnail.jp
A strong-coupling analysis of two-dimensional O(N) sigma models with on square, triangular and honeycomb lattices
Recently-generated long strong-coupling series for the two-point Green's
functions of asymptotically free lattice models are
analyzed, focusing on the evaluation of dimensionless renormalization-group
invariant ratios of physical quantities and applying resummation techniques to
series in the inverse temperature and in the energy . Square,
triangular, and honeycomb lattices are considered, as a test of universality
and in order to estimate systematic errors. Large- solutions are carefully
studied in order to establish benchmarks for series coefficients and
resummations. Scaling and universality are verified. All invariant ratios
related to the large-distance properties of the two-point functions vary
monotonically with , departing from their large- values only by a few per
mille even down to .Comment: 53 pages (incl. 5 figures), tar/gzip/uuencode, REVTEX + psfi
Polydisperse Adsorption: Pattern Formation Kinetics, Fractal Properties, and Transition to Order
We investigate the process of random sequential adsorption of polydisperse
particles whose size distribution exhibits a power-law dependence in the small
size limit, . We reveal a relation between pattern
formation kinetics and structural properties of arising patterns. We propose a
mean-field theory which provides a fair description for sufficiently small
. When , highly ordered structures locally identical
to the Apollonian packing are formed. We introduce a quantitative criterion of
the regularity of the pattern formation process. When , a sharp
transition from irregular to regular pattern formation regime is found to occur
near the jamming coverage of standard random sequential adsorption with
monodisperse size distribution.Comment: 8 pages, LaTeX, 5 figures, to appear in Phys.Rev.
Universality of low-energy scattering in (2+1) dimensions
We prove that, in (2+1) dimensions, the S-wave phase shift, , k
being the c.m. momentum, vanishes as either as . The constant is universal and .
This result is established first in the framework of the Schr\"odinger equation
for a large class of potentials, second for a massive field theory from proved
analyticity and unitarity, and, finally, we look at perturbation theory in
and study its relation to our non-perturbative result. The
remarkable fact here is that in n-th order the perturbative amplitude diverges
like as , while the full amplitude vanishes as . We show how these two facts can be reconciled.Comment: 23 pages, Late
Personalized pathology maps to quantify diffuse and focal brain damage.
Quantitative MRI (qMRI) permits the quantification of brain changes compatible with inflammation, degeneration and repair in multiple sclerosis (MS) patients. In this study, we propose a new method to provide personalized maps of tissue alterations and longitudinal brain changes based on different qMRI metrics, which provide complementary information about brain pathology.
We performed baseline and two-years follow-up on (i) 13 relapsing-remitting MS patients and (ii) four healthy controls. A group consisting of up to 65 healthy controls was used to compute the reference distribution of qMRI metrics in healthy tissue. All subjects underwent 3T MRI examinations including T1, T2, T2* relaxation and Magnetization Transfer Ratio (MTR) imaging. We used a recent partial volume estimation algorithm to estimate the concentration of different brain tissue types on T1 maps; then, we computed a deviation map (z-score map) for each contrast at both time-points. Finally, we subtracted those deviation maps only for voxels showing a significant difference with healthy tissue in one of the time points, to obtain a difference map for each subject.
Control subjects did not show any significant z-score deviations or longitudinal z-score changes. On the other hand, MS patients showed brain regions with cross-sectional and longitudinal concomitant increase in T1, T2, T2* z-scores and decrease of MTR z-scores, suggesting brain tissue degeneration/loss. In the lesion periphery, we observed areas with cross-sectional and longitudinal decreased T1/T2 and slight decrease in T2* most likely related to iron accumulation. Moreover, we measured longitudinal decrease in T1, T2 - and to a lesser extent in T2* - as well as a concomitant increase in MTR, suggesting remyelination/repair. In summary, we have developed a method that provides whole-brain personalized maps of cross-sectional and longitudinal changes in MS patients, which are computed in patient space. These maps may open new perspectives to complement and support radiological evaluation of brain damage for a given patient
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COVID-19-related stigma among infected people in Sweden; psychometric properties and levels of stigma in two cohorts as measured by a COVID-19 stigma scale
BACKGROUND: Epidemics have historically been accompanied by stigma and discrimination. Disease-related stigma has often been shown to have severe consequences for physical, mental and social wellbeing and lead to barriers to diagnosis, treatment and prevention. The aims of this study were to investigate if a HIV-related stigma measure could be adapted and valid and reliable to measure COVID-19-related stigma, and also to investigate levels of self-reported stigma and related factors among people in Sweden with experience of COVID-19 and compare levels of COVID-19-related stigma versus HIV-related stigma among persons living with HIV who had experienced a COVID-19 event.
METHODS: Cognitive interviews (n = 11) and cross-sectional surveys were made after the acute phase of the illness using a new 12-item COVID-19 Stigma Scale and the established 12-item HIV Stigma Scale in two cohorts (people who had experienced COVID-19 (n = 166/209, 79%) and people living with HIV who had experienced a COVID-19 event (n = 50/91, 55%). Psychometric analysis of the COVID-19 Stigma Scale was performed by calculating floor and ceiling effects, Cronbach's α and exploratory factor analysis. Levels of COVID-19 stigma between groups were analysed using the Mann-Whitney U test. Levels of COVID-19 and HIV stigma among people living with HIV with a COVID-19 event were compared using the Wilcoxon signed-rank test.
RESULTS: The COVID-19 cohort consisted of 88 (53%) men and 78 (47%) women, mean age 51 (19-80); 143 (87%) living in a higher and 22 (13%) in a lower income area. The HIV + COVID-19 cohort consisted of 34 (68%) men and 16 (32%) women, mean age 51 (26-79); 20 (40%) living in a higher and 30 (60%) in a lower income area. The cognitive interviews showed that the stigma items were easy to understand. Factor analysis suggested a four-factor solution accounting for 77% of the total variance. There were no cross loadings, but two items loaded on factors differing from the original scale. All subscales had acceptable internal consistency, showed high floor and no ceiling effects. There was no statistically significant difference between COVID-19 stigma scores between the two cohorts or between genders. People living in lower income areas reported more negative self-image and concerns about public attitudes related to COVID-19 than people in higher income areas (median score 3 vs 3 and 4 vs 3 on a scale from 3-12, Z = -1.980, p = 0.048 and Z = -2.023, p = 0.024, respectively). People from the HIV + COVID-19 cohort reported more HIV than COVID-19 stigma.
CONCLUSIONS: The adapted 12-item COVID-19 Stigma Scale may be valid and reliable for measurement of COVID-19-related stigma. However, specific items may need to be rephrased or replaced to better correspond to the COVID-19 context. People who had experienced COVID-19 reported low levels of COVID-19-related stigma in general but people from lower income areas had higher levels of negative self-image and concerns about public attitudes related to COVID-19 than people from areas with higher income, which may call for targeted interventions. Although exhibiting more pronounced HIV stigma levels, people living with HIV who had experienced COVID-19 reported COVID-19-related stigma of the same low magnitude as their peers not living with HIV
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