30,333 research outputs found
Surface Critical Behavior in Systems with Absorbing States
We present a general scaling theory for the surface critical behavior of
non-equilibrium systems with phase transitions into absorbing states. The
theory allows for two independent surface exponents which satisfy generalized
hyperscaling relations. As an application we study a generalized version of
directed percolation with two absorbing states. We find two distinct surface
universality classes associated with inactive and reflective walls. Our results
indicate that the exponents associated with these two surface universality
classes are closely connected.Comment: latex, 4 pages, to appear in PR
Persistence in the Voter model: continuum reaction-diffusion approach
We investigate the persistence probability in the Voter model for dimensions
d\geq 2. This is achieved by mapping the Voter model onto a continuum
reaction-diffusion system. Using path integral methods, we compute the
persistence probability r(q,t), where q is the number of ``opinions'' in the
original Voter model. We find r(q,t)\sim exp[-f_2(q)(ln t)^2] in d=2;
r(q,t)\sim exp[-f_d(q)t^{(d-2)/2}] for 2<d<4; r(q,t)\sim exp[-f_4(q)t/ln t] in
d=4; and r(q,t)\sim exp[-f_d(q)t] for d>4. The results of our analysis are
checked by Monte Carlo simulations.Comment: 10 pages, 3 figures, Latex, submitted to J. Phys. A (letters
From urban to national heat island: The effect of anthropogenic heat output on climate change in high population industrial countries
The project presented here sought to determine whether changes in anthropogenic thermal emission can have a measurable effect on temperature at the national level, taking Japan and Great Britain as type examples. Using energy consumption as a proxy for thermal emission, strong correlations (mean r2 = 0.90 and 0.89, respectively) are found between national equivalent heat output (HO) and temperature above background levels Δt averaged over 5‐ to 8‐yr periods between 1965 and 2013, as opposed to weaker correlations for CMIP5 model temperatures above background levels Δmt (mean r2 = 0.52 and 0.10). It is clear that the fluctuations in Δt are better explained by energy consumption than by present climate models, and that energy consumption can contribute to climate change at the national level on these timescales
When it Pays to Rush: Interpreting Morphogen Gradients Prior to Steady-State
During development, morphogen gradients precisely determine the position of
gene expression boundaries despite the inevitable presence of fluctuations.
Recent experiments suggest that some morphogen gradients may be interpreted
prior to reaching steady-state. Theoretical work has predicted that such
systems will be more robust to embryo-to-embryo fluctuations. By analysing two
experimentally motivated models of morphogen gradient formation, we investigate
the positional precision of gene expression boundaries determined by
pre-steady-state morphogen gradients in the presence of embryo-to-embryo
fluctuations, internal biochemical noise and variations in the timing of
morphogen measurement. Morphogens that are direct transcription factors are
found to be particularly sensitive to internal noise when interpreted prior to
steady-state, disadvantaging early measurement, even in the presence of large
embryo-to-embryo fluctuations. Morphogens interpreted by cell-surface receptors
can be measured prior to steady-state without significant decrease in
positional precision provided fluctuations in the timing of measurement are
small. Applying our results to experiment, we predict that Bicoid, a
transcription factor morphogen in Drosophila, is unlikely to be interpreted
prior to reaching steady-state. We also predict that Activin in Xenopus and
Nodal in zebrafish, morphogens interpreted by cell-surface receptors, can be
decoded in pre-steady-state.Comment: 18 pages, 3 figure
Rheology of Active Filament Solutions
We study the viscoelasticity of an active solution of polar biofilaments and
motor proteins. Using a molecular model, we derive the constitutive equations
for the stress tensor in the isotropic phase and in phases with liquid
crystalline order. The stress relaxation in the various phases is discussed.
Contractile activity is responsible for a spectacular difference in the
viscoelastic properties on opposite sides of the order-disorder transition.Comment: 4 pages, 1 figur
Where we stand on structure dependence of ISGMR in the Zr-Mo region: Implications on K_\infty
Isoscalar giant resonances, being the archetypal forms of collective nuclear
behavior, have been studied extensively for decades with the goal of
constraining bulk nuclear properties of the equation of state, as well as for
modeling dynamical behaviors within stellar environments. An important such
mode is the isoscalar electric giant monopole resonance (ISGMR) that can be
understood as a radially symmetric density vibration within the saturated
nuclear volume. The field has a few key open questions, which have been
proposed and remain unresolved. One of the more provocative questions is the
extra high-energy strength in the region, which manifested in
large percentages of the sum rule in Zr and Mo above the
main ISGMR peak. The purpose of this article is to introduce these questions
within the context of experimental investigations into the phenomena in the
zirconium and molybdenum isotopic chains, and to address, via a discussion of
previously published and preliminary results, the implications of recent
experimental efforts on extraction of the nuclear incompressibility from this
data.Comment: 9 pages, 7 figures, invited to be submitted to a special issue of
EPJA honoring Prof. P. F. Bortigno
Simulation model of erosion and deposition on a barchan dune
Erosion and deposition over a barchan dune near the Salton Sea, California, are modeled by bookkeeping the quantity of sand in saltation following streamlines of transport. Field observations of near surface wind velocity and direction plus supplemental measurements of the velocity distribution over a scale model of the dune are combined as input to Bagnold type sand transport formulas corrected for slope effects. A unidirectional wind is assumed. The resulting patterns of erosion and deposition compare closely with those observed in the field and those predicted by the assumption of equilibrium (downwind translation of the dune without change in size or geometry). Discrepancies between the simulated results and the observed or predicted erosional patterns appear to be largely due to natural fluctuations in the wind direction. The shape of barchan dunes is a function of grain size, velocity, degree of saturation of the oncoming flow, and the variability in the direction of the oncoming wind. The size of the barchans may be controlled by natural atmospheric scales, by the age of the dunes, or by the upwind roughness. The upwind roughness can be controlled by fixed elements or by sand in the saltation. In the latter case, dune scale is determined by grain size and wind velocity
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