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arXiv:cond-mat/9810105 · Journal of Physics A Mathematical and General

Nonuniversality in short-time critical dynamics

César Simões, J. R. Drugowich de Felício

Published Sep 4, 1998• 8 citations
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Abstract

We study the behaviour of dynamical critical exponents of the two-dimensional Ising model with a line of defects. Simulations done at an early time (first 100 Monte Carlo steps) reveal that the critical exponent of Janssen et al ( Z. Phys. B 73 539) depends on the strength of the exchange coupling constant of the altered line. On the other hand, our simulations permit us to conclude that the dynamical critical exponent z is not sensitive to changes in . In addition, we investigate the possible invariance of the anomalous dimension of the magnetization at the beginning of the process.

DOI
10.1088/0305-4470/31/35/007

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We study the behaviour of dynamical critical exponents of the two-dimensional Ising model with a line of defects. Simulations done at an early time (first 100 Monte Carlo steps) reveal that the critical exponent of Janssen et al ( Z. Phys. B 73 539) depends on the strength of the exchange coupling constant of the altered line. On the other hand, our simulations permit us to conclude that the dynamical critical exponent z is not sensitive to changes in . In addition, we investigate the possible invariance of the anomalous dimension of the magnetization at the beginning of the process.
We consider two-dimensional Ising models with randomly distributed ferromagnetic bonds and study the local critical behavior at defect lines by extensive Monte Carlo simulations. Both for ladder and chain type defects, non-universal critical behavior is observed: the critical exponent of the defect magnetization is found to be a continuous function of the strength of the defect coupling. Analyzing corresponding stability conditions, we obtain new evidence that the critical exponent ννν of the bulk correlation length of the random Ising model does not depend on dilution, i.e. ν=1ν=1ν=1.
Comprehensive Monte Carlo simulations of the short-time dynamic behaviour are reported for the three-dimensional Ising model at criticality. Besides the exponent of the critical initial increase and the dynamic exponent z , the static critical exponents and as well as the critical temperature are determined from the power-law scaling behaviour of observables at the beginning of the time evolution. States of very high temperature as well as of zero temperature are used as initial states for the simulations.
The time evolution of the three-dimensional critical Ising model relaxing from a non-equilibrium initial state is studied by means of Monte Carlo simulation. We observe the characteristic initial increase of the (spatially) averaged magnetization predicted by Janssen et al. The exponent theta ' that governs the initial behaviour is determined, and the dependence of the long-time linear decay on the initial magnetization, m 0 , is analysed. Our simulation corroborates earlier results derived from continuum models.
Monte Carlo simulations of critical dynamics has been used to study the short-time process and scaling behavior of two-layer Ising model. The result shows that there are critical points even when the coupling between the two layers is not equal to zero; the short-time dynamic critical exponent θ was determined; the traditional critical exponent 1/νz was estimated also.Our result encouraged the supposition of the existence of the critical line and evidenced that this model was likely a weak universal model.
With Monte Carlo simulations, we study the dynamic relaxation at perfect and imperfect surfaces of the three-dimensional Ising model with an ordered initial state. The time evolution of the surface magnetization, the line magnetization of the defect line, and the corresponding susceptibilities and second cumulants is carefully examined. Universal dynamic scaling forms including a dynamic crossover scaling form are identified at the ordinary, special, and surface phase transitions. The critical exponents beta1 of the surface magnetization and beta2 of the line magnetization are extracted. The impact of the defect line on the universality classes is investigated.