866 research outputs found
Critical Behavior and Lack of Self Averaging in the Dynamics of the Random Potts Model in Two Dimensions
We study the dynamics of the q-state random bond Potts ferromagnet on the
square lattice at its critical point by Monte Carlo simulations with single
spin-flip dynamics. We concentrate on q=3 and q=24 and find, in both cases,
conventional, rather than activated, dynamics. We also look at the distribution
of relaxation times among different samples, finding different results for the
two q values. For q=3 the relative variance of the relaxation time tau at the
critical point is finite. However, for q=24 this appears to diverge in the
thermodynamic limit and it is ln(tau) which has a finite relative variance. We
speculate that this difference occurs because the transition of the
corresponding pure system is second order for q=3 but first order for q=24.Comment: 9 pages, 13 figures, final published versio
Probability distributions of the work in the 2D-Ising model
Probability distributions of the magnetic work are computed for the 2D Ising
model by means of Monte Carlo simulations. The system is first prepared at
equilibrium for three temperatures below, at and above the critical point. A
magnetic field is then applied and grown linearly at different rates.
Probability distributions of the work are stored and free energy differences
computed using the Jarzynski equality. Consistency is checked and the dynamics
of the system is analyzed. Free energies and dissipated works are reproduced
with simple models. The critical exponent is estimated in an usual
manner.Comment: 12 pages, 6 figures. Comments are welcom
Test of Local Scale Invariance from the direct measurement of the response function in the Ising model quenched to and to below
In order to check on a recent suggestion that local scale invariance
[M.Henkel et al. Phys.Rev.Lett. {\bf 87}, 265701 (2001)] might hold when the
dynamics is of Gaussian nature, we have carried out the measurement of the
response function in the kinetic Ising model with Glauber dynamics quenched to
in , where Gaussian behavior is expected to apply, and in the two
other cases of the model quenched to and to below , where
instead deviations from Gaussian behavior are expected to appear. We find that
in the case there is an excellent agreement between the numerical data,
the local scale invariance prediction and the analytical Gaussian
approximation. No logarithmic corrections are numerically detected. Conversely,
in the cases, both in the quench to and to below , sizable
deviations of the local scale invariance behavior from the numerical data are
observed. These results do support the idea that local scale invariance might
miss to capture the non Gaussian features of the dynamics. The considerable
precision needed for the comparison has been achieved through the use of a fast
new algorithm for the measurement of the response function without applying the
external field. From these high quality data we obtain for
the scaling exponent of the response function in the Ising model quenched
to below , in agreement with previous results.Comment: 24 pages, 6 figures. Resubmitted version with improved discussions
and figure
Logarithmic corrections in the aging of the fully-frustrated Ising model
We study the dynamics of the critical two-dimensional fully-frustrated Ising
model by means of Monte Carlo simulations. The dynamical exponent is estimated
at equilibrium and is shown to be compatible with the value . In a
second step, the system is prepared in the paramagnetic phase and then quenched
at its critical temperature . Numerical evidences for the existence of
logarithmic corrections in the aging regime are presented. These corrections
may be related to the topological defects observed in other fully-frustrated
models. The autocorrelation exponent is estimated to be as for the
Ising chain quenched at .Comment: 12 pages, 9 figure
On universality in aging ferromagnets
This work is a contribution to the study of universality in
out-of-equilibrium lattice models undergoing a second-order phase transition at
equilibrium. The experimental protocol that we have chosen is the following:
the system is prepared in its high-temperature phase and then quenched at the
critical temperature . We investigated by mean of Monte Carlo simulations
two quantities that are believed to take universal values: the exponent
obtained from the decay of autocorrelation functions and the
asymptotic value of the fluctuation-dissipation ratio . This
protocol was applied to the Ising model, the 3-state clock model and the
4-state Potts model on square, triangular and honeycomb lattices and to the
Ashkin-Teller model at the point belonging at equilibrium to the 3-state Potts
model universality class and to a multispin Ising model and the Baxter-Wu model
both belonging to the 4-state Potts model universality class at equilibrium.Comment: 17 page
Disorder driven phase transitions of the large q-state Potts model in 3d
Phase transitions induced by varying the strength of disorder in the large-q
state Potts model in 3d are studied by analytical and numerical methods. By
switching on the disorder the transition stays of first order, but different
thermodynamical quantities display essential singularities. Only for strong
enough disorder the transition will be soften into a second-order one, in which
case the ordered phase becomes non-homogeneous at large scales, while the
non-correlated sites percolate the sample. In the critical regime the critical
exponents are found universal: \beta/\nu=0.60(2) and \nu=0.73(1).Comment: 4 pages; 3 figure
Clinical studies, the interests and limits of using dabigatran in atrial fibrillation
Atrial fibrillation (AF) is the most frequent cardiac arrhythmia, especially in older people. This condition is associated with an increased risk of stroke, and long-term anticoagulation treatment is therefore needed. Vitamin K antagonists are effective in reducing the risk of stroke but optimal use of these drugs remains difficult. The development of new oral anticoagulant drugs is therefore highly relevant. Dabigatran is an oral direct thrombin inhibitor. Its prodrug, dabigatran etexilate, is marketed under the name of Pradaxa and was initially approved for the prevention of thromboembolic events in major orthopedic surgery. It has been recently approved for stroke prevention in patients with AF. The purpose of this paper is to review--in light of current knowledge--the interests and limits of using dabigatran etexilate in AF. Briefly, dabigatran etexilate is not inferior to warfarin in AF. However many questions remain unanswered, including questions related to the concomitant use of dabigatran etexilate and acetylsalicylic acid, the possible increased risk of myocardial infarction and the need for drug monitoring
Aging, memory and rejuvenation: some lessons from simple models
Many recent experiments probed the off equilibrium dynamics of spin glasses
and other glassy systems through temperature cycling protocols and observed
memory and rejuvenation phenomena. Here we show through numerical simulations,
using powerful algorithms, that such features can already be observed to some
extent in simple models such as two dimensional ferromagnets. We critically
discuss these results and review some aspects of the literature in the light of
our findings.Comment: 10 pages, 8 figures. Contribution to the Proceedings of the
Summerschool "Ageing and the glass transition", Luxembourg 14-25 Sept. 200
Fluctuation relations in non-equilibrium stationary states of Ising models
Fluctuation relations for the entropy production in non equilibrium
stationary states of Ising models are investigated by Monte Carlo simulations.
Systems in contact with heat baths at two different temperatures or subject to
external driving will be studied. In the first case, by considering different
kinetic rules and couplings with the baths, the behavior of the probability
distributions of the heat exchanged in a time with the thermostats, both
in the disordered and in the low temperature phase, are discussed. The
fluctuation relation is always verified in the large limit and
deviations from linear response theory are observed. Finite- corrections
are shown to obey a scaling behavior. In the other case the system is in
contact with a single heat bath but work is done by shearing it. Also for this
system the statistics collected for the mechanical work shows the validity of
the fluctuation relation and preasymptotic corrections behave analogously to
the case with two baths.Comment: 9 figure
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