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Lithuanian Journal of Physics

On ergodic relaxation time in the three-dimensional Ising model

Robertas Grigalaits, Saulius Lapinskas, J. Banys, E. E. Tornau

Published Jan 1, 2013• 1 citation
Publisher

Abstract

We have studied the dynamical decay of the autocorrelation function of the 3D Ising model for different sizes L = 20–52 of spin cluster-cubes. The behaviour of the longest, ergodic relaxation time, τe, of a finite domain below the phase transition temperature Tc was mostly considered for two types of phase transition dynamics. A study of the scaling properties of τe demonstrates a negligible difference between the types of dynamics used, but a considerable difference for different boundary conditions. In contrast to the known result for periodic boundary conditions (τe ~ Lz exp [const(Lєν)2], where z and ν are the dynamical and correlation length exponents, respectively, and є = 1 – T/Tc), the ergodic relaxation time for open boundary conditions is proportional to Lz exp [const(Lєν)2k] with coeffcient k for lattices explored in this work slightly decreasing with L in between 1.65 and 1.58. This result implies that only the lattices of sizes close to or exceeding L = 300 with open boundary conditions might have ergodic relaxation times similar to those with perodic boundary conditions.

DOI
10.3952/lithjphys.53305

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We have studied the dynamical decay of the autocorrelation function of the 3D Ising model for different sizes L = 20–52 of spin cluster-cubes. The behaviour of the longest, ergodic relaxation time, τe, of a finite domain below the phase transition temperature Tc was mostly considered for two types of phase transition dynamics. A study of the scaling properties of τe demonstrates a negligible difference between the types of dynamics used, but a considerable difference for different boundary conditions. In contrast to the known result for periodic boundary conditions (τe ~ Lz exp [const(Lєν)2], where z and ν are the dynamical and correlation length exponents, respectively, and є = 1 – T/Tc), the ergodic relaxation time for open boundary conditions is proportional to Lz exp [const(Lєν)2k] with coeffcient k for lattices explored in this work slightly decreasing with L in between 1.65 and 1.58. This result implies that only the lattices of sizes close to or exceeding L = 300 with open boundary conditions might have ergodic relaxation times similar to those with perodic boundary conditions.
The Ising model (and also the Ising model in a transverse field) has been used extensively to investigate ferroelectric phase transitions. Numerical Monte Carlo simulations of phase transitions in three-dimensional Ising systems with variable size (2 ≤ L ≤ 100) have been performed. The susceptibility maximum and peak temperature Tc 3D(L,Lc) for free and periodic boundary conditions have been determinated and have been observed to be well fitted by simple expressions as a function of L. For L → ∞, as expected [Tc 3D]fbc = [Tc 3D]pbc being fbc = free boundary conditions and pbc = periodic boundary conditions. Scaling plots MLβ/v vs |ϵ|Ll/v for larger L with both boundary conditions were obteined and compared. The scaling function for our data up to L ≈ 100 was examined closely and an excellent fit at T → Tc was obtenied.
We report the results of a Monte Carlo investigation of the (equilibrium) time-displaced correlation functions for the magnetization and energy of a simple cubic Ising model as a function of time, temperature, and lattice size. The simulations were carried out on a CDC CYBER 205 supercomputer employing a high-speed, vectorized multispin coding program and using a total of 5\ifmmode\times\else\texttimes\fi{}1012{10}^{12}1012 Monte Carlo spin-flip trials. We used L\ifmmode\times\else\texttimes\fi{}L\ifmmode\times\else\texttimes\fi{}L lattices with periodic boundary conditions and L as large as 96. The short-time and long-time behaviors of the correlation functions are analyzed by fits to a sum of exponential decays, and the critical exponent z for the largest relaxation time is extracted using a finite-size-scaling analysis. Our estimate z=2.04\ifmmode\pm\else\textpm\fi{}0.03 resolves an intriguing contradiction in the literature; it satisfies the theoretical lower bound and is in agreement with the prediction obtained by \ensuremath{\epsilon} expansion. We also consider various small systematic errors that typically occur in the analysis of relaxation functions and show how they can lead to spurious results if sufficient care is not exercised.
The modified definition of the correlation length for the four dimensional Ising model \xi \propto \epsilon^{-\nu} log^{1/6}\epsilon [1] predicts that the relaxation time for a quantity near the critical temperature T_c should diverge as \tau \propto \epsilon^{-\Delta} log^{\Delta/3}\epsilon where \epsilon, \nu, and \Delta are, respectively, the reduced temperature ( \epsilon=( T_c-T )/T_c, T_c = 6.68 ), the correlation length critical exponent, and a dynamical critical exponent. The \tau-data for the nonlinear relaxation of the order parameter computed on the Creutz cellular automaton [2,3], by using the finite-size lattices of linear dimension L = 12 and 14 as approximations to the infinite lattice for T.
We have carried out extensive Monte Carlo simulations of the dynamic critical behavior of L\ifmmode\times\else\texttimes\fi{}L\ifmmode\times\else\texttimes\fi{}L simple-cubic classical Heisenberg models with periodic boundary conditions. The equilibrium relaxation time \ensuremath{\tau} is determined from the long-time exponential decay of the time-displaced correlation function of the magnetization. Using finite-size scaling and corrections to scaling we find that at the critical point Tc{\mathit{T}}_{\mathit{c}}Tc​=(0.6929${)}^{\mathrm{\ensuremath{-}}1}$, \ensuremath{\tau} grows like \ensuremath{\tau}\ensuremath{\simeq}Lz{\mathit{L}}^{\mathit{z}}Lz with z=1.96(6). In the paramagnetic region we show that the relaxation time is represented by a universal scaling function \ensuremath{\tau}${\mathit{L}}^{\mathrm{\ensuremath{-}}\mathit{z}}$ vs (T/Tc{\mathit{T}}_{\mathit{c}}Tc​-1)${\mathit{L}}^{1/\ensuremath{\nu}}$ with z=1.96(6). This estimate is consistent with theoretical predictions and results of experimental studies concerning the dynamical critical behavior of Heisenberg magnets perturbed by order-parameter nonconserving dipolar interactions. We also describe how a simple over-relaxation algorithm of Creutz and of Brown and Woch can be incorporated into the Metropolis sampling method for the classical Heisenberg model, thereby significantly reducing the critical slowing down; the resultant correlation time varies as \ensuremath{\tau}\ensuremath{\sim}(T/Tc{\mathit{T}}_{\mathit{c}}Tc​-1${)}^{\mathrm{\ensuremath{-}}0.77}$, which implies z\ensuremath{\simeq}1.10.
I present an analysis of the dynamic behavior of short-range Ising spin glasses observed in stochastic simulations. The time dependence of the order parameter q(t)=〈Sx{S}_{x}Sx​(0)Sx{S}_{x}Sx​(t)〉\ifmmode\bar\else\textasciimacron\fi{}---which is the same as that of the structure factor---and the time dependence of the related dynamic correlation functions have been recorded with good statistics and very long observation times. The spin-glass model with a symmetric distribution of discrete nearest-neighbor \ifmmode\pm\else\textpm\fi{}J interactions on a simple-cubic lattice was used. Simulations were performed with a special fast computer, allowing for the first-time investigation of the equilibrium dynamics for a wide range of temperatures (0.7\ensuremath{\le}kT/J\ensuremath{\le}5.0) and lattice sizes (83{8}^{3}83, 163{16}^{3}163, 323{32}^{3}323, and 643{64}^{3}643). I have found that the empirical formula q(t)=${\mathrm{ct}}^{\mathrm{\ensuremath{-}}x}$exp(-\ensuremath{\omega}${t}^{\ensuremath{\beta}}$) with temperature-dependent exponents x(T) and \ensuremath{\beta}(T) describes the decay very well at all temperatures above the spin-glass transition. In the spin-glass phase, only the algebraic decay q(t)=${\mathrm{ct}}^{\mathrm{\ensuremath{-}}x}$ could be observed, with different temperature dependences of the exponent x(T). The dynamic scaling hypothesis and finite-size scaling explain well the observed temperature and size dependence of the data, and the functional form of the correlation functions is com- patible with the scaling form if corrections to scaling are taken into account. The scaling behavior and the dynamic and static critical exponents found in my simulations are in reasonable agreement with recent experiments performed on insulating spin glasses, showing that despite its simplicity the discrete model of spin glasses analyzed in this work displays behavior similar to that seen in nature.
Employing Monte Carlo simulations, the equilibrium dynamic critical behavior or L×L×L simple cubic classical Heisenberg models with periodic boundary conditions are determined. From the long-time exponential decay of the time-displaced correlation function of the magnetization the linear (equilibrium) relaxation time τ is also determined. It is found that at the critical point Tc, τ grows like τ≂Lz with z=1.94±0.06. In the paramagnetic region a power-law divergence τ≂(T/Tc−1)−Δ was found with Δ=zν=1.38±0.04, which implies a dynamical critical exponent z=1.96±0.07. Both values are consistent with theoretical predictions and results of experimental studies concerning the dynamical critical behavior of Heisenberg magnets perturbed by order-parameter nonconserving forces.