Physical review. E
Monte Carlo static and dynamic simulations of a three-dimensional Ising critical model
F. Livet
Abstract
The critical dynamics of `model A'' of Hohenberg and Halperin has been studied by the Monte Carlo method. Simulations have been carried out in the three-dimensional (3d) simple cubic Ising model for lattices of sizes $L=16$ to $L=512$. Using Wolff's cluster algorithm, the critical temperature is precisely found as ${\ensuremath{\beta}}_{c}=0.221\phantom{\rule{0.16em}{0ex}}654\phantom{\rule{0.16em}{0ex}}68(5)$. By Fourier transform of the lattice configurations, the critical scattering intensities $I(\stackrel{P\vec}{q}\phantom{\rule{0.16em}{0ex}})$ can be obtained. After circular averaging, the static simulations with $L=256$ and $L=512$ provide an estimate of the critical exponent $\ensuremath{\gamma}/\ensuremath{\nu}=2\ensuremath{-}\ensuremath{\eta}=1.9640(5)$. The $|\stackrel{P\vec}{q}\phantom{\rule{0.16em}{0ex}}|$-dependent distribution of $I(\stackrel{P\vec}{q}\phantom{\rule{0.16em}{0ex}})$ showed an exponential distribution, corresponding to a Gaussian distribution of the scattering amplitudes for a large $q$ domain. The time-dependent intensities were then used for the study of the critical dynamics of 3d lattices at the critical point. To simulate results of an x-ray photon correlation spectroscopy experiment, the time-dependent correlation function of the intensities was studied for each $|\stackrel{P\vec}{q}\phantom{\rule{0.16em}{0ex}}|$-value. In the $q$ region where $I(\stackrel{P\vec}{q}\phantom{\rule{0.16em}{0ex}})$ had an exponential distribution, the time correlations can be fit to a stretched exponential, where the exponent $\ensuremath{\mu}=\ensuremath{\gamma}/\ensuremath{\nu}z\ensuremath{\simeq}0.975$ provides an estimate of the dynamic exponent $z$. This corresponds to $z=2.0145$, in agreement with the observed variations of the characteristic fluctuation time of the intensity: $\ensuremath{\tau}(q)\ensuremath{\propto}{q}^{\ensuremath{-}z}$, which gives $z=2.015$. These results agree with the $\ensuremath{\epsilon}$ expansion of field-theoretical methods (2.017). In this paper, the need to take account of the anomalous time behavior ($\ensuremath{\mu}<1$) in the dynamics is exemplified. This dynamics reflects a nonlinear time behavior of model A, and its large time extension is discussed in detail.