4,325 research outputs found
Unexpected Effect of Internal Degrees of Freedom on Transverse Phonons in Supercooled Liquids
We show experimentally that in a supercooled liquid composed of molecules
with internal degrees of freedom the internal modes contribute to the frequency
dependent shear viscosity and damping of transverse phonons, which results in
an additional broadening of the transverse Brillouin lines. Earlier, only the
effect of internal modes on the frequency dependent bulk viscosity and damping
of longitudinal phonons was observed and explained theoretically in the limit
of weak coupling of internal degrees of freedom to translational motion. A new
theory is needed to describe this new effect. We also demonstrate, that the
contributions of structural relaxation and internal processes to the width of
the Brillouin lines can be separated by measurements under high pressure
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Evaluation of Non-photorealistic 3D Urban Models for Mobile Device Navigation.
Genome-wide analysis points to roles for extracellular matrix remodeling, the visual cycle, and neuronal development in myopia
Myopia, or nearsightedness, is the most common eye disorder, resulting
primarily from excess elongation of the eye. The etiology of myopia, although
known to be complex, is poorly understood. Here we report the largest ever
genome-wide association study (43,360 participants) on myopia in Europeans. We
performed a survival analysis on age of myopia onset and identified 19
significant associations (p < 5e-8), two of which are replications of earlier
associations with refractive error. These 19 associations in total explain 2.7%
of the variance in myopia age of onset, and point towards a number of different
mechanisms behind the development of myopia. One association is in the gene
PRSS56, which has previously been linked to abnormally small eyes; one is in a
gene that forms part of the extracellular matrix (LAMA2); two are in or near
genes involved in the regeneration of 11-cis-retinal (RGR and RDH5); two are
near genes known to be involved in the growth and guidance of retinal ganglion
cells (ZIC2, SFRP1); and five are in or near genes involved in neuronal
signaling or development. These novel findings point towards multiple genetic
factors involved in the development of myopia and suggest that complex
interactions between extracellular matrix remodeling, neuronal development, and
visual signals from the retina may underlie the development of myopia in
humans
Efficient Replication of Over 180 Genetic Associations with Self-Reported Medical Data
While the cost and speed of generating genomic data have come down dramatically in recent years, the slow pace of collecting medical data for large cohorts continues to hamper genetic research. Here we evaluate a novel online framework for amassing large amounts of medical information in a recontactable cohort by assessing our ability to replicate genetic associations using these data. Using web-based questionnaires, we gathered self-reported data on 50 medical phenotypes from a generally unselected cohort of over 20,000 genotyped individuals. Of a list of genetic associations curated by NHGRI, we successfully replicated about 75% of the associations that we expected to (based on the number of cases in our cohort and reported odds ratios, and excluding a set of associations with contradictory published evidence). Altogether we replicated over 180 previously reported associations, including many for type 2 diabetes, prostate cancer, cholesterol levels, and multiple sclerosis. We found significant variation across categories of conditions in the percentage of expected associations that we were able to replicate, which may reflect systematic inflation of the effects in some initial reports, or differences across diseases in the likelihood of misdiagnosis or misreport. We also demonstrated that we could improve replication success by taking advantage of our recontactable cohort, offering more in-depth questions to refine self-reported diagnoses. Our data suggests that online collection of self-reported data in a recontactable cohort may be a viable method for both broad and deep phenotyping in large populations
Fabrication of large-area CCD detectors on high-purity, float-zone silicon
In this report on the fabrication of a 1024 x 1024 charge coupled device (CCD) imager to be used as a soft x-ray sensor onboard the Advanced X-ray Astronomical Facility (AXAF), the following conclusions were found: the dislocations that limited the performance of the high resistivity imager were characterized; the sources of stress were identified and the dislocations found were eliminated; and a charge transfer inefficiency (CTI) of 10(exp -6) and read noise as low as 1.3/e was demonstrated. This sensor must have low noise and a low CTI and must be radiation hardened to withstand any radiation damage from a space environment
Searching for Earth analogues around the nearest stars: the disk age-metallicity relation and the age distribution in the Solar Neighbourhood
The chemical composition of Earth's atmosphere has undergone substantial
evolution over the course of its history. It is possible, even likely, that
terrestrial planets in other planetary systems have undergone similar changes;
consequently, the age distribution of nearby stars is an important
consideration in designing surveys for Earth-analogues. Valenti & Fischer
(2005) provide age and metallicity estimates for 1039 FGK dwarfs in the Solar
Neighbourhood. Using the Hipparcos catalogue as a reference to calibrate
potential biases, we have extracted volume-limited samples of nearby stars from
the Valenti-Fischer dataset. Unlike other recent investigations, our analysis
shows clear evidence for an age-metallicity relation in the local disk, albeit
with substantial dispersion at any epoch. The mean metallicity increases from
-0.3 dex at a lookback time of ~10 Gyrs to +0.15 dex at the present day.
Supplementing the Valenti-Fischer measurements with literature data to give a
complete volume-limited sample, the age distribution of nearby FGK dwarfs is
broadly consistent with a uniform star-formation rate over the history of the
Galactic disk. In striking contrast, most stars known to have planetary
companions are younger than 5 Gyrs; however, stars with planetary companions
within 0.4 AU have a significantly flatter age distribution, indicating that
those systems are stable on timescales of many Gyrs. Several of the older,
lower metallicity host stars have enhanced [alpha/Fe] ratios, implying
membership of the thick disk. If the frequency of terrestrial planets is also
correlated with stellar metallicity, then the median age of such planetary
system is likely to be ~3 Gyrs. We discuss the implications of this hypothesis
in designing searches for Earth analogues among the nearby stars.Comment: Accepted for publication in Ap
Growing Correlation Length on Cooling Below the Onset of Caging in a Simulated Glass-Forming Liquid
We present a calculation of a fourth-order, time-dependent density
correlation function that measures higher-order spatiotemporall correlations of
the density of a liquid. From molecular dynamics simulations of a glass-forming
Lennard-Jones liquid, we find that the characteristic length scale of this
function has a maximum as a function of time which increases steadily beyond
the characteristic length of the static pair correlation function in the
temperature range approaching the mode coupling temperature from above
Hydrophobic and ionic-interactions in bulk and confined water with implications for collapse and folding of proteins
Water and water-mediated interactions determine thermodynamic and kinetics of
protein folding, protein aggregation and self-assembly in confined spaces. To
obtain insights into the role of water in the context of folding problems, we
describe computer simulations of a few related model systems. The dynamics of
collapse of eicosane shows that upon expulsion of water the linear hydrocarbon
chain adopts an ordered helical hairpin structure with 1.5 turns. The structure
of dimer of eicosane molecules has two well ordered helical hairpins that are
stacked perpendicular to each other. As a prelude to studying folding in
confined spaces we used simulations to understand changes in hydrophobic and
ionic interactions in nano droplets. Solvation of hydrophobic and charged
species change drastically in nano water droplets. Hydrophobic species are
localized at the boundary. The tendency of ions to be at the boundary where
water density is low increases as the charge density decreases. Interaction
between hydrophobic, polar, and charged residue are also profoundly altered in
confined spaces. Using the results of computer simulations and accounting for
loss of chain entropy upon confinement we argue and then demonstrate, using
simulations in explicit water, that ordered states of generic amphiphilic
peptide sequences should be stabilized in cylindrical nanopores
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
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