571 research outputs found
Percolation of randomly distributed growing clusters
We investigate the problem of growing clusters, which is modeled by two
dimensional disks and three dimensional droplets. In this model we place a
number of seeds on random locations on a lattice with an initial occupation
probability, . The seeds simultaneously grow with a constant velocity to
form clusters. When two or more clusters eventually touch each other they
immediately stop their growth. The probability that such a system will result
in a percolating cluster depends on the density of the initially distributed
seeds and the dimensionality of the system. For very low initial values of
we find a power law behavior for several properties that we investigate, namely
for the size of the largest and second largest cluster, for the probability for
a site to belong to the finally formed spanning cluster, and for the mean
radius of the finally formed droplets. We report the values of the
corresponding scaling exponents. Finally, we show that for very low initial
concentration of seeds the final coverage takes a constant value which depends
on the system dimensionality.Comment: 5 pages, 7 figure
Percolation of randomly distributed growing clusters: Finite Size Scaling and Critical Exponents
We study the percolation properties of the growing clusters model. In this
model, a number of seeds placed on random locations on a lattice are allowed to
grow with a constant velocity to form clusters. When two or more clusters
eventually touch each other they immediately stop their growth. The model
exhibits a discontinuous transition for very low values of the seed
concentration and a second, non-trivial continuous phase transition for
intermediate values. Here we study in detail this continuous transition
that separates a phase of finite clusters from a phase characterized by the
presence of a giant component. Using finite size scaling and large scale Monte
Carlo simulations we determine the value of the percolation threshold where the
giant component first appears, and the critical exponents that characterize the
transition. We find that the transition belongs to a different universality
class from the standard percolation transition.Comment: 5 two-column pages, 6 figure
Multiscale model of electronic behavior and localization in stretched dry DNA
When the DNA double helix is subjected to external forces it can stretch elastically to elongations reaching 100% of its natural length. These distortions, imposed at the mesoscopic or macroscopic scales, have a dramatic effect on electronic properties at the atomic scale and on electrical transport along DNA. Accordingly, a multiscale approach is necessary to capture the electronic behavior of the stretched DNA helix. To construct such a model, we begin with accurate density-functional-theory calculations for electronic states in DNA bases and base pairs in various relative configurations encountered in the equilibrium and stretched forms. These results are complemented by semi-empirical quantum mechanical calculations for the states of a small size [18 base pair poly(CG)–poly(CG)] dry, neutral DNA sequence, using previously published models for stretched DNA. The calculated electronic states are then used to parametrize an effective tight-binding model that can describe electron hopping in the presence of environmental effects, such as the presence of stray water molecules on the backbone or structural features of the substrate. These effects introduce disorder in the model hamiltonian which leads to electron localization. The localization length is smaller by several orders of magnitude in stretched DNA relative to that in the unstretched structure
Soil-Structure Interaction Effects on the Response of Meloland Bridge
A study related to the experimental data obtained from the full-scale tests of Meloland Road overcrossing is presented. The major objective of this study is to demonstrate that the soil structure interaction effects play a significant role in the dynamic response of the structure. Initially, the results from the static deformation of the bridge under the quick-release loads are presented. These results were obtained by applying a special integration technique on the experimental acceleration records. Then a preliminary analytical model of the bridge is fitted to the experimental data in order to evaluate the values of the soil-structure springs along with other parameters of the bridge. Finally, a Finite Element model for the foundation of the pier was developed in order to obtain the load- (or strain-) dependent values of the pier foundation stiffnesses. These values are compared to those obtained from the bridge model
MicroRNA profiling reveals marker of motor neuron disease in ALS models
Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disorder marked by the loss of motor neurons (MNs) in the brain and spinal cord, leading to fatally debilitating weakness. Because this disease predominantly affects MNs, we aimed to characterize the distinct expression profile of that cell type to elucidate underlying disease mechanisms and to identify novel targets that inform on MN health during ALS disease time course. microRNAs (miRNAs) are short, noncoding RNAs that can shape the expression profile of a cell and thus often exhibit cell-type-enriched expression. To determine MN-enriched miRNA expression, we used Cre recombinase-dependent miRNA tagging and affinity purification in mice. By defining thein vivomiRNA expression of MNs, all neurons, astrocytes, and microglia, we then focused on MN-enriched miRNAs via a comparative analysis and found that they may functionally distinguish MNs postnatally from other spinal neurons. Characterizing the levels of the MN-enriched miRNAs in CSF harvested from ALS models of MN disease demonstrated that one miRNA (miR-218) tracked with MN loss and was responsive to an ALS therapy in rodent models. Therefore, we have used cellular expression profiling tools to define the distinct miRNA expression of MNs, which is likely to enrich future studies of MN disease. This approach enabled the development of a novel, drug-responsive marker of MN disease in ALS rodents.SIGNIFICANCE STATEMENTAmyotrophic lateral sclerosis (ALS) is a neurodegenerative disease in which motor neurons (MNs) in the brain and spinal cord are selectively lost. To develop tools to aid in our understanding of the distinct expression profiles of MNs and, ultimately, to monitor MN disease progression, we identified small regulatory microRNAs (miRNAs) that were highly enriched or exclusive in MNs. The signal for one of these MN-enriched miRNAs is detectable in spinal tap biofluid from an ALS rat model, where its levels change as disease progresses, suggesting that it may be a clinically useful marker of disease status. Furthermore, rats treated with ALS therapy have restored expression of this MN RNA marker, making it an MN-specific and drug-responsive marker for ALS rodents.</jats:p
Time-dependent calculation of ionization in Potassium at mid-infrared wavelengths
We study the dynamics of the Potassium atom in the mid-infrared, high
intensity, short laser pulse regime. We ascertain numerical convergence by
comparing the results obtained by the direct expansion of the time-dependent
Schroedinger equation onto B-Splines, to those obtained by the eigenbasis
expansion method. We present ionization curves in the 12-, 13-, and 14-photon
ionization range for Potassium. The ionization curve of a scaled system, namely
Hydrogen starting from the 2s, is compared to the 12-photon results. In the
13-photon regime, a dynamic resonance is found and analyzed in some detail. The
results for all wavelengths and intensities, including Hydrogen, display a
clear plateau in the peak-heights of the low energy part of the Above Threshold
Ionization (ATI) spectrum, which scales with the ponderomotive energy Up, and
extends to 2.8 +- 0.5 Up.Comment: 15 two-column pages with 15 figures, 3 tables. Accepted for
publication in Phys. Rev A. Improved figures, language and punctuation, and
made minor corrections. We also added a comparison to the ADK theor
Multidrug resistant Acinetobacter baumannii: a descriptive study in a city hospital
<p>Abstract</p> <p>Background</p> <p>Multidrug resistant <it>Acinetobacter baumannii</it>, (MRAB) is an important cause of hospital acquired infection. The purpose of this study is to determine the risk factors for MRAB in a city hospital patient population.</p> <p>Methods</p> <p>This study is a retrospective review of a city hospital epidemiology data base and includes 247 isolates of Acinetobacter baumannii (AB) from 164 patients. Multidrug resistant <it>Acinetobacter baumannii </it>was defined as resistance to more than three classes of antibiotics. Using the non-MRAB isolates as the control group, the risk factors for the acquisition of MRAB were determined.</p> <p>Results</p> <p>Of the 247 AB isolates 72% (177) were multidrug resistant. Fifty-eight percent (143/247) of isolates were highly resistant (resistant to imipenem, amikacin, and ampicillin-sulbactam). Of the 37 patients who died with Acinetobacter colonization/infection, 32 (86%) patients had the organism recovered from the respiratory tract. The factors which were found to be significantly associated (p ≤ 0.05) with multidrug resistance include the recovery of AB from multiple sites, mechanical ventilation, previous antibiotic exposure, and the presence of neurologic impairment. Multidrug resistant Acinetobacter was associated with significant mortality when compared with sensitive strains (p ≤ 0.01). When surgical patients (N = 75) were considered separately, mechanical ventilation and multiple isolates remained the factors significantly associated with the development of multidrug resistant Acinetobacter. Among surgical patients 46/75 (61%) grew a multidrug resistant strain of AB and 37/75 (40%) were resistant to all commonly used antibiotics including aminoglycosides, cephalosporins, carbepenems, extended spectrum penicillins, and quinolones. Thirty-five percent of the surgical patients had AB cultured from multiple sites and 57% of the Acinetobacter isolates were associated with a co-infecting organism, usually a Staphylococcus or Pseudomonas. As in medical patients, the isolation of Acinetobacter from multiple sites and the need for mechanical ventilation were significantly associated with the development of MRAB.</p> <p>Conclusions</p> <p>The factors significantly associated with MRAB in both the general patient population and surgical patients were mechanical ventilation and the recovery of Acinetobacter from multiple anatomic sites. Previous antibiotic use and neurologic impairment were significant factors in medical patients. Colonization or infection with MRAB is associated with increased mortality.</p
Many-electron tunneling in atoms
A theoretical derivation is given for the formula describing N-electron
ionization of atom by a dc field and laser radiation in tunneling regime.
Numerical examples are presented for noble gases atoms.Comment: 11 pages, 1 EPS figure, submitted to JETP (Jan 99
- …