222 research outputs found
Monte Carlo studies of antiferromagnetic spin models in three dimensions
We study several antiferromagnetic formulations of the O(3) spin model in
three dimensions by means of Monte Carlo simulations. We discuss about the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first orderComment: 4 pages, 2 Postscript figures. Contribution to Lattice '9
A Cluster Method for the Ashkin--Teller Model
A cluster Monte Carlo algorithm for the Ashkin-Teller (AT) model is
constructed according to the guidelines of a general scheme for such
algorithms. Its dynamical behaviour is tested for the square lattice AT model.
We perform simulations on the line of critical points along which the exponents
vary continuously, and find that critical slowing down is significantly
reduced. We find continuous variation of the dynamical exponent along the
line, following the variation of the ratio , in a manner which
satisfies the Li-Sokal bound , that was so far
proved only for Potts models.Comment: 18 pages, Revtex, figures include
FATORES AMBIENTAIS SOBRE A IDADE AO PRIMEIRO PARTO, DIAS ABERTOS E INTERVALO ENTRE PARTOS EM VACAS DA RAĂA HOLANDESA NA BACIA LEITEIRA DE CASTROLANDA, ESTADO DO PARANĂ
Dados provenientes do Programa de AnĂĄlise de Rebanhos Leiteiros do ParanĂĄ (PARLPR) da
Associação Paranaense de Criadores de Bovinos Leiteiros da Raça Holandesa (APCBRH) foram analisados
para estudar os fatores do meio ambiente (rebanho, ano de parto, mĂȘs de parto, grupo genĂ©tico, idade ao parto
e efeito vaca) que estariam influenciando as caracterĂsticas reprodutivas: idade ao primeiro parto (IPP) em
meses, dias abertos (DA) e intervalo entre partos (IEP) em dias, nas vacas da raça Holandesa na Bacia Leiteira
de Castrolanda, Castro, Estado do ParanĂĄ. Para o estudo dos efeitos de meio ambiente sobre a idade ao
primeiro parto, foram utilizadas 10.494 primĂparas da raça Holandesa, variedade HPB (Preta e Branca),
pertencentes a 68 rebanhos, controlados entre 1991 e 2000. Para os dias abertos e intervalo entre partos,
foram utilizadas 16.232 vacas, pertencentes a 67 rebanhos, das mesmas raça e variedade. As médias e os
respectivos desvios-padrão observados para o IPP, DA e IEP, foram: 27,05 ± 3,93 meses, 98,73 ± 33,03 dias e
380,73 ± 33,03 dias, respectivamente. Para as anĂĄlises estatĂsticas dos dados foi utilizado o MĂ©todo GLS
(General Least Square), pelo Proc Mixed do programa SASÂź, versĂŁo 6.1. Os efeitos de rebanho, ano de parto,
mĂȘs de parto e grupo genĂ©tico foram altamente significativos (
Finite Size Scaling and ``perfect'' actions: the three dimensional Ising model
Using Finite-Size Scaling techniques, we numerically show that the first
irrelevant operator of the lattice theory in three dimensions
is (within errors) completely decoupled at . This interesting
result also holds in the Thermodynamical Limit, where the renormalized coupling
constant shows an extraordinary reduction of the scaling-corrections when
compared with the Ising model. It is argued that Finite-Size Scaling analysis
can be a competitive method for finding improved actions.Comment: 13 pages, 3 figure
New Universality Class in three dimensions: the Antiferromagnetic model
We present the results of a Monte Carlo simulation of the model in
three dimensions with negative coupling. We observe a second order phase
transition between the disordered phase and an antiferromagnetic, unfrustrated,
ordered one. We measure, with a Finite Size Scaling analysis, the thermal
exponent, obtaining . We have found two magnetic-type relevant
operators whose related exponents are and
respectively.Comment: 10 pages, 2 Postscript figures. Revised version: references adde
Universality Class of Thermally Diluted Ising Systems at Criticality
The universality class of thermally diluted Ising systems, in which the
realization of the disposition of magnetic atoms and vacancies is taken from
the local distribution of spins in the pure original Ising model at
criticality, is investigated by finite size scaling techniques using the Monte
Carlo method. We find that the critical temperature, the critical exponents and
therefore the universality class of these thermally diluted Ising systems
depart markedly from the ones of short range correlated disordered systems. Our
results agree fairly well with theoretical predictions previously made by
Weinrib and Halperin for systems with long range correlated disorder.Comment: 7 pages, 6 figures, RevTe
Coarse-grained loop algorithms for Monte Carlo simulation of quantum spin systems
Recently, Syljuasen and Sandvik proposed a new framework for constructing
algorithms of quantum Monte Carlo simulation. While it includes new classes of
powerful algorithms, it is not straightforward to find an efficient algorithm
for a given model. Based on their framework, we propose an algorithm that is a
natural extension of the conventional loop algorithm with the split-spin
representation. A complete table of the vertex density and the worm-scattering
probability is presented for the general XXZ model of an arbitrary S with a
uniform magnetic field.Comment: 12 pages, 7 figures, insert a word "squared" in the first line of the
caption of Fig.7 and correct the label of vertical axis of Fig.
Ising spins coupled to a four-dimensional discrete Regge skeleton
Regge calculus is a powerful method to approximate a continuous manifold by a
simplicial lattice, keeping the connectivities of the underlying lattice fixed
and taking the edge lengths as degrees of freedom. The discrete Regge model
employed in this work limits the choice of the link lengths to a finite number.
To get more precise insight into the behavior of the four-dimensional discrete
Regge model, we coupled spins to the fluctuating manifolds. We examined the
phase transition of the spin system and the associated critical exponents. The
results are obtained from finite-size scaling analyses of Monte Carlo
simulations. We find consistency with the mean-field theory of the Ising model
on a static four-dimensional lattice.Comment: 19 pages, 7 figure
Cluster Monte Carlo Simulations of the Nematic--Isotropic Transition
We report the results of simulations of the Lebwohl-Lasher model of the
nematic-isotropic transition using a new cluster Monte Carlo algorithm. The
algorithm is a modification of the Wolff algorithm for spin systems, and
greatly reduces critical slowing down. We calculate the free energy in the
neighborhood of the transition for systems up to linear size 70. We find a
double well structure with a barrier that grows with increasing system size,
obeying finite size scaling for systems of size greater than 35. We thus obtain
an estimate of the value of the transition temperature in the thermodynamic
limit.Comment: 4 figure
Finite size effects on measures of critical exponents in d=3 O(N) models
We study the critical properties of three-dimensional O(N) models, for
N=2,3,4. Parameterizing the leading corrections-to-scaling for the
exponent, we obtain a reliable infinite volume extrapolation, incompatible with
previous Monte Carlo values, but in agreement with -expansions. We
also measure the critical exponent related with the tensorial magnetization as
well as the exponents and critical couplings.Comment: 12 pages, 2 postscript figure
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