12,732 research outputs found
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
Radio-quiet and radio-loud pulsars: similar in Gamma-rays but different in X-rays
We present new Chandra and XMM-Newton observations of a sample of eight
radio-quiet Gamma-ray pulsars detected by the Fermi Large Area Telescope. For
all eight pulsars we identify the X-ray counterpart, based on the X-ray source
localization and the best position obtained from Gamma-ray pulsar timing. For
PSR J2030+4415 we found evidence for an about 10 arcsec-long pulsar wind
nebula. Our new results consolidate the work from Marelli et al. 2011 and
confirm that, on average, the Gamma-ray--to--X-ray flux ratios (Fgamma/Fx) of
radio-quiet pulsars are higher than for the radio-loud ones. Furthermore, while
the Fgamma/Fx distribution features a single peak for the radio-quiet pulsars,
the distribution is more dispersed for the radio-loud ones, possibly showing
two peaks. We discuss possible implications of these different distributions
based on current models for pulsar X-ray emission.Comment: Accepted for publication in The Astrophysical Journal; 12 pages, 3
figures, 2 table
A Swendsen-Wang update algorithm for the Symanzik improved sigma model
We study a generalization of Swendsen-Wang algorithm suited for Potts models
with next-next-neighborhood interactions. Using the embedding technique
proposed by Wolff we test it on the Symanzik improved bidimensional non-linear
model. For some long range observables we find a little slowing down
exponent () that we interpret as an effect of the partial
frustration of the induced spin model.Comment: Self extracting archive fil
Energy Distribution in disordered elastic Networks
Disordered networks are found in many natural and artificial materials, from gels or cytoskeletal structures to metallic foams or bones. Here, the energy distribution in this type of networks is modeled, taking into account the orientation of the struts. A correlation between the orientation and the energy per unit volume is found and described as a function of the connectivity in the network and the relative bending stiffness of the struts. If one or both parameters have relatively large values, the struts aligned in the loading direction present the highest values of energy. On the contrary, if these have relatively small values, the highest values of energy can be reached in the struts oriented transversally. This result allows explaining in a simple way remodeling processes in biological materials, for example, the remodeling of trabecular bone and the reorganization in the cytoskeleton. Additionally, the correlation between the orientation, the affinity, and the bending-stretching ratio in the network is discussed
Gradual Certified Programming in Coq
Expressive static typing disciplines are a powerful way to achieve
high-quality software. However, the adoption cost of such techniques should not
be under-estimated. Just like gradual typing allows for a smooth transition
from dynamically-typed to statically-typed programs, it seems desirable to
support a gradual path to certified programming. We explore gradual certified
programming in Coq, providing the possibility to postpone the proofs of
selected properties, and to check "at runtime" whether the properties actually
hold. Casts can be integrated with the implicit coercion mechanism of Coq to
support implicit cast insertion a la gradual typing. Additionally, when
extracting Coq functions to mainstream languages, our encoding of casts
supports lifting assumed properties into runtime checks. Much to our surprise,
it is not necessary to extend Coq in any way to support gradual certified
programming. A simple mix of type classes and axioms makes it possible to bring
gradual certified programming to Coq in a straightforward manner.Comment: DLS'15 final version, Proceedings of the ACM Dynamic Languages
Symposium (DLS 2015
The Tails of the Crossing Probability
The scaling of the tails of the probability of a system to percolate only in
the horizontal direction was investigated numerically for correlated
site-bond percolation model for .We have to demonstrate that the
tails of the crossing probability far from the critical point have shape
where is the correlation
length index, is the probability of a bond to be closed. At
criticality we observe crossover to another scaling . Here is a scaling index describing the
central part of the crossing probability.Comment: 20 pages, 7 figures, v3:one fitting procedure is changed, grammatical
change
Tests of the continuum limit for the Principal Chiral Model and the prediction for \L_\MS
We investigate the continuum limit in Principal Chiral Models
concentrating in detail on the model and its covering group
SU(2)xSU(2). We compute the mass gap in terms of Lambda_MS and compare with the
prediction of Hollowood of m/\L_\MS = 3.8716. We use the finite-size scaling
method of L\"uscher et al. to deduce m/\L_\MS and find that for the
model the computed result of m/\L_\MS \sim 14 is in strong disagreement with
theory but that a similar analysis of the SU(2)xSU(2) yields excellent
agreement with theory. We conjecture that for violations of the
finite-size scaling assumption are severe forthe values of the correlation
length, , investigated and that our attempts to extrapolate the results to
zero lattice spacing, although plausible, are erroneous. Conversely, the
finite-size scaling violations in the SU(2)xSU(2) simulation are consistent
with perturbation theory and the computed function agrees well with the
3-loop approximation for couplings evaluated at scales , where
is measured in units of the lattice spacing, . We conjecture that
lattice vortex artifacts in the model are responsible for delaying the
onset of the continuum limit until much larger correlation lengths are achieved
notwithstanding the apparent onset of scaling. Results for the mass spectrum
for SO(N) m, N=8,10 are given whose comparison with theory gives plausible
support to our ideas.Comment: 27 pages , 1 Postscript-file, uuencode
Cluster algorithms
Cluster algorithms for classical and quantum spin systems are discussed. In
particular, the cluster algorithm is applied to classical O(N) lattice actions
containing interactions of more than two spins. The performance of the
multi-cluster and single--cluster methods, and of the standard and improved
estimators are compared. (Lecture given at the summer school on `Advances in
Computer Simulations', Budapest, July 1996.)Comment: 17 pages, Late
Testing fixed points in the 2D O(3) non-linear sigma model
Using high statistic numerical results we investigate the properties of the
O(3) non-linear 2D sigma-model. Our main concern is the detection of an
hypothetical Kosterlitz-Thouless-like (KT) phase transition which would
contradict the asymptotic freedom scenario. Our results do not support such a
KT-like phase transition.Comment: Latex, 7 pgs, 4 eps-figures. Added more analysis on the
KT-transition. 4-loop beta function contains corrections from D.-S.Shin
(hep-lat/9810025). In a note-added we comment on the consequences of these
corrections on our previous reference [16
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