3,451 research outputs found
Dynamic and static properties of the invaded cluster algorithm
Simulations of the two-dimensional Ising and 3-state Potts models at their
critical points are performed using the invaded cluster (IC) algorithm. It is
argued that observables measured on a sub-lattice of size l should exhibit a
crossover to Swendsen-Wang (SW) behavior for l sufficiently less than the
lattice size L, and a scaling form is proposed to describe the crossover
phenomenon. It is found that the energy autocorrelation time tau(l,L) for an
l*l sub-lattice attains a maximum in the crossover region, and a dynamic
exponent z for the IC algorithm is defined according to tau_max ~ L^z.
Simulation results for the 3-state model yield z=.346(.002) which is smaller
than values of the dynamic exponent found for the SW and Wolff algorithms and
also less than the Li-Sokal bound. The results are less conclusive for the
Ising model, but it appears that z<.21 and possibly that tau_max ~ log L so
that z=0 -- similar to previous results for the SW and Wolff algorithms.Comment: 21 pages with 12 figure
Dynamic Critical Behavior of the Swendsen-Wang Algorithm: The Two-Dimensional 3-State Potts Model Revisited
We have performed a high-precision Monte Carlo study of the dynamic critical
behavior of the Swendsen-Wang algorithm for the two-dimensional 3-state Potts
model. We find that the Li-Sokal bound ()
is almost but not quite sharp. The ratio seems to diverge
either as a small power () or as a logarithm.Comment: 35 pages including 3 figures. Self-unpacking file containing the
LaTeX file, the needed macros (epsf.sty, indent.sty, subeqnarray.sty, and
eqsection.sty) and the 3 Postscript figures. Revised version fixes a
normalization error in \xi (with many thanks to Wolfhard Janke for finding
the error!). To be published in J. Stat. Phys. 87, no. 1/2 (April 1997
The ability of hop extracts to reduce the methane production of methanobrevibacter ruminantium
Background. Methane emissions from agriculture are responsible for over 40% of the world’s greenhouse gas emissions. In the past, antibiotics were used to control methane production by animals, but concerns over the emergence and spread of antibiotic-resistant bacteria to humans have prompted a search for alternative approaches. Hops are the flowers of the hop plant Humulus lupulus. They have been used to feed cattle for many years and are known to contain antibacterial compounds, and their extracts have been shown to kill members of the Mycobacterium spp including Mycobacterium bovis, the causative agent of bovine tuberculosis as well as a number of human pathogens. In this study, hop extracts were studied for their ability to inhibit methane production from Methanobrevibacter ruminantium, a major methane-producing archaeon found in the rumen of cattle. Methods. Methanobrevibacter ruminantium M1T (DSM 1093) was grown at 37°C for 30 days, and the amount of methane produced at different time points during this period was measured using gas chromatography. The archaeon was exposed to commercial hop extracts (tetra-hydro-iso-alpha acid and beta acid) and to aqueous hop extracts of a range of hop variants, and their effect on methane production was determined. Results. All of the extracts reduced the level of methane production of M. ruminantium over the 30-day period compared to the negative control (sterile distilled water). The commercial hop extracts were the most effective at inhibiting methane production over the course of the experiment in contrast to the aqueous extracts, which showed a gradual reduction of inhibition with time. Conclusions. Hops contain compounds which inhibit methane production. Given that hops can be safely fed to cattle, this raises the possibility of rationally designing a feed strategy which could reduce greenhouse gas emissions and protect against bovine tuberculosis. This study recommends that further research be undertaken to further identifying bioactive components from hops and their efficacy against a range of archaea
Critical Behavior of Random Bond Potts Models
The effect of quenched impurities on systems which undergo first-order phase
transitions is studied within the framework of the q-state Potts model. For
large q a mapping to the random field Ising model is introduced which provides
a simple physical explanation of the absence of any latent heat in 2D, and
suggests that in higher dimensions such systems should exhibit a tricritical
point with a correlation length exponent related to the exponents of the random
field model by \nu = \nu_RF / (2 - \alpha_RF - \beta_RF). In 2D we analyze the
model using finite-size scaling and conformal invariance, and find a continuous
transition with a magnetic exponent \beta / \nu which varies continuously with
q, and a weakly varying correlation length exponent \nu \approx 1. We find
strong evidence for the multiscaling of the correlation functions as expected
for such random systems.Comment: 13 pages, RevTeX. 4 figures included. Submitted to Phys.Rev.Let
Cluster variation - Pade` approximants method for the simple cubic Ising model
The cluster variation - Pade` approximant method is a recently proposed tool,
based on the extrapolation of low/high temperature results obtained with the
cluster variation method, for the determination of critical parameters in
Ising-like models. Here the method is applied to the three-dimensional simple
cubic Ising model, and new results, obtained with an 18-site basic cluster, are
reported. Other techniques for extracting non-classical critical exponents are
also applied and their results compared with those by the cluster variation -
Pade` approximant method.Comment: 8 RevTeX pages, 3 PostScript figure
Dynamic Critical Behavior of a Swendsen-Wang-Type Algorithm for the Ashkin-Teller Model
We study the dynamic critical behavior of a Swendsen-Wang-type algorithm for
the Ashkin--Teller model. We find that the Li--Sokal bound on the
autocorrelation time ()
holds along the self-dual curve of the symmetric Ashkin--Teller model, and is
almost but not quite sharp. The ratio appears
to tend to infinity either as a logarithm or as a small power (). In an appendix we discuss the problem of extracting estimates of
the exponential autocorrelation time.Comment: 59 pages including 3 figures, uuencoded g-compressed ps file.
Postscript size = 799740 byte
Chorea-related mutations in PDE10A result in aberrant compartmentalization and functionality of the enzyme
A robust body of evidence supports the concept that phosphodiesterase 10A (PDE10A) activity in the basal ganglia orchestrates the control of coordinated movement in human subjects. Although human mutations in the PDE10A gene manifest in hyperkinetic movement disorders that phenocopy many features of early Huntington’s disease, characterization of the maladapted molecular mechanisms and aberrant signaling processes that underpin these conditions remains scarce. Recessive mutations in the GAF-A domain have been shown to impair PDE10A function due to the loss of striatal PDE10A protein levels, but here we show that this paucity is caused by irregular intracellular trafficking and increased PDE10A degradation in the cytosolic compartment. In contrast to GAF-A mutants, dominant mutations in the GAF-B domain of PDE10A induce PDE10A misfolding, a common pathological phenotype in many neurodegenerative diseases. These data demonstrate that the function of striatal PDE10A is compromised in disorders where disease-associated mutations trigger a reduction in the fidelity of PDE compartmentalization
Monte Carlo Renormalization of the 3-D Ising model: Analyticity and Convergence
We review the assumptions on which the Monte Carlo renormalization technique
is based, in particular the analyticity of the block spin transformations. On
this basis, we select an optimized Kadanoff blocking rule in combination with
the simulation of a d=3 Ising model with reduced corrections to scaling. This
is achieved by including interactions with second and third neighbors. As a
consequence of the improved analyticity properties, this Monte Carlo
renormalization method yields a fast convergence and a high accuracy. The
results for the critical exponents are y_H=2.481(1) and y_T=1.585(3).Comment: RevTeX, 4 PostScript file
Computing the Roughening Transition of Ising and Solid-On-Solid Models by BCSOS Model Matching
We study the roughening transition of the dual of the 2D XY model, of the
Discrete Gaussian model, of the Absolute Value Solid-On-Solid model and of the
interface in an Ising model on a 3D simple cubic lattice. The investigation
relies on a renormalization group finite size scaling method that was proposed
and successfully tested a few years ago. The basic idea is to match the
renormalization group flow of the interface observables with that of the
exactly solvable BCSOS model. Our estimates for the critical couplings are
, and for
the XY-model, the Discrete Gaussian model and the Absolute Value Solid-On-Solid
model, respectively. For the inverse roughening temperature of the Ising
interface we find . To the best of our knowledge,
these are the most precise estimates for these parameters published so far.Comment: 25 pages, LaTeX file, no figure
Evaluation of stem rot in 339 Bornean tree species: implications of size, taxonomy, and soil-related variation for aboveground biomass estimates
Fungal decay of heart wood creates hollows and areas of reduced wood density within the stems of living trees known as stem rot. Although stem rot is acknowledged as a source of error in forest aboveground biomass (AGB) estimates, there are few data sets available to evaluate the controls over stem rot infection and severity in tropical forests. Using legacy and recent data from 3180 drilled, felled, and cored stems in mixed dipterocarp forests in Sarawak, Malaysian Borneo, we quantified the frequency and severity of stem rot in a total of 339 tree species, and related variation in stem rot with tree size, wood density, taxonomy, and species’ soil association, as well as edaphic conditions. Predicted stem rot frequency for a 50 cm tree was 53% of felled, 39% of drilled, and 28% of cored stems, demonstrating differences among methods in rot detection ability. The percent stem volume infected by rot, or stem rot severity, ranged widely among trees with stem rot infection (0.1–82.8 %) and averaged 9% across all trees felled. Tree taxonomy explained the greatest proportion of variance in both stem rot frequency and severity among the predictors evaluated in our models. Stem rot frequency, but not severity, increased sharply with tree diameter, ranging from 13% in trees 10–30 cm DBH to 54%in stems ≥ 50 cm DBH across all data sets. The frequency of stem rot increased significantly in soils with low pH and cation concentrations in topsoil, and stem rot was more common in tree species associated with dystrophic sandy soils than with nutrient-rich clays. When scaled to forest stands, the maximum percent of stem biomass lost to stem rot varied significantly with soil properties, and we estimate that stem rot reduces total forest AGB estimates by up to 7% relative to what would be predicted assuming all stems are composed strictly of intact wood. This study demonstrates not only that stem rot is likely to be a significant source of error in forest AGB estimation, but also that it strongly covaries with tree size, taxonomy, habitat association, and soil resources, underscoring the need to account for tree community composition and edaphic variation in estimating carbon storage in tropical forests
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