Monte Carlo simulation of universal short-time behaviour in critical relaxation

Critical short-time behavior of majority-vote model on scale-free networks

We discuss the short-time behavior of the majority vote dynamics on scale-free networks at the critical threshold. A heterogeneous mean-field theory on the critical short-time behavior of the majority-vote model on scale-free networks is introduced. In addition, the heterogeneous mean-field predictions are compared with extensive Monte Carlo simulations of the short-time dependencies of the order parameter and the susceptibility. Closed expressions of the dynamical exponent z and the time correlation exponent ν_{∥} are obtained. The short-time scaling is compatible with a nonuniversal critical behavior for 5/2<γ<7/2. However, for γ≥7/2, we have the mean-field Ising criticality with additional logarithmic corrections for γ=7/2, the same as the stationary scaling.

Nonequilibrium critical dynamics of the two-dimensional Ashkin-Teller model at the Baxter line

We investigate the short-time universal behavior of the two-dimensional Ashkin-Teller model at the Baxter line by performing time-dependent Monte Carlo simulations. First, as preparatory results, we obtain the critical parameters by searching the optimal power-law decay of the magnetization. Thus, the dynamic critical exponents θ_{m} and θ_{p}, related to the magnetic and electric order parameters, as well as the persistence exponent θ_{g}, are estimated using heat-bath Monte Carlo simulations. In addition, we estimate the dynamic exponent z and the static critical exponents β and ν for both order parameters. We propose a refined method to estimate the static exponents that considers two different averages: one that combines an internal average using several seeds with another, which is taken over temporal variations in the power laws. Moreover, we also performed the bootstrapping method for a complementary analysis. Our results show that the ratio β/ν exhibits universal behavior along the critical line corroborating the conjecture for both magnetization and polarization.

Generalized dynamic scaling for quantum critical relaxation in imaginary time

We study the imaginary-time relaxation critical dynamics of a quantum system with a vanishing initial correlation length and an arbitrary initial order parameter M0. We find that in quantum critical dynamics, the behavior of M0 under scale transformations deviates from a simple power law, which was proposed for very small M0 previously. A universal characteristic function is then suggested to describe the rescaled initial magnetization, similar to classical critical dynamics. This characteristic function is shown to be able to describe the quantum critical dynamics in both short- and long-time stages of the evolution. The one-dimensional transverse-field Ising model is employed to numerically determine the specific form of the characteristic function. We demonstrate that it is applicable as long as the system is in the vicinity of the quantum critical point. The universality of the characteristic function is confirmed by numerical simulations of models belonging to the same universality class.

Short-time critical dynamics of damage spreading in the two-dimensional Ising model

The short-time critical dynamics of propagation of damage in the Ising ferromagnet in two dimensions is studied by means of Monte Carlo simulations. Starting with equilibrium configurations at T=∞ and magnetization M=0 , an initial damage is created by flipping a small amount of spins in one of the two replicas studied. In this way, the initial damage is proportional to the initial magnetization M0 in one of the configurations upon quenching the system at T C, the Onsager critical temperature of the ferromagnetic-paramagnetic transition. It is found that, at short times, the damage increases with an exponent θ D=1.915(3) , which is much larger than the exponent θ=0.197 characteristic of the initial increase of the magnetization M(t). Also, an epidemic study was performed. It is found that the average distance from the origin of the epidemic (R2(t)) grows with an exponent z∗ ≈ η ≈ 1.9, which is the same, within error bars, as the exponent θ D. However, the survival probability of the epidemics reaches a plateau so that δ=0. On the other hand, by quenching the system to lower temperatures one observes the critical spreading of the damage at T D ≃ 0.51TC, where all the measured observables exhibit power laws with exponents θ D=1.026(3), δ=0.133(1), and z∗=1.74(3).

Logarithmic correction to scaling in domain-wall dynamics at Kosterlitz-Thouless phase transitions

With Monte Carlo simulations, we investigate the relaxation dynamics of domain walls at the Kosterlitz-Thouless phase transition, taking the two-dimensional XY model as an example. The dynamic scaling behavior is carefully analyzed, and a domain-wall roughening process is observed. Two-time correlation functions are calculated and aging phenomena are investigated. Inside the domain interface, a strong logarithmic correction to scaling is detected.

Nonequilibrium critical relaxation of the order parameter and energy in the two-dimensional ferromagnetic Potts model

The static and dynamic critical properties of the ferromagnetic q -state Potts models on a square lattice with q=2 and 3 are numerically studied via the nonequilibrium relaxation method. The relaxation behavior of both the order parameter and energy as well as that of the second moments are investigated, from which static and dynamic critical exponents can be obtained. We find that the static exponents thus obtained from the relaxation of the order parameter and energy together with the second moments of the order parameter exhibit a close agreement with the exact exponents, especially for the case of the q=2 (Ising) model, when care is taken in the choice of the initial states for the relaxation of the second moments. As for the case of q=3 , the estimates for the static exponents become less accurate, but still exhibit reasonable agreement with the exactly known static exponents. The dynamic critical exponent for the q=2 (Ising) model is estimated from the relaxation of the second moments of the order parameter with mixed initial conditions to give z(q=2) approximately 2.1668(19) .

Nonequilibrium critical dynamics with domain wall and surface

With Monte Carlo simulations, we investigate the relaxation dynamics with a domain wall for magnetic systems at the critical temperature. The dynamic scaling behavior is carefully analyzed, and a dynamic roughening process is observed. For comparison, similar analysis is applied to the relaxation dynamics with a free or disordered surface.

Short-time critical dynamics and aging phenomena in the two-dimensional XY model

With Monte Carlo methods, we simulate dynamic relaxation processes of the two-dimensional XY model at the Berezinskii-Kosterlitz-Thouless phase transition temperature and below, starting from both ordered and disordered initial states. The two-time correlation function A(t',t) is measured, and aging phenomena are investigated. The power-law correction in the spatial correlation length xi(t) for relaxation with an ordered initial state and the logarithmic correction for relaxation with a disordered initial state are carefully analyzed. The scaling functions of A(t',t) are then extracted.

Dynamic Monte Carlo simulations of the three-dimensional random-bond Potts model

The effect of random bonds on the phase transitions of the three-dimensional three-state Potts model is investigated with extensive dynamic Monte Carlo simulations. In the weakly disordered regime, the phase diagram is obtained with a recently suggested nonequilibrium reweighting method. The tricritical point separating the first- and second-order transitions is determined, and the critical exponents of the continuous phase transition induced by quenched randomness are estimated.

Continuous majority-vote model

We introduce a kinetic irreversible XY model and investigate its dynamic critical behavior through short-time Monte Carlo simulations on square lattices with periodic boundary conditions, starting from an ordered state. We find evidence that this system exhibits a Kosterlitz-Thouless-like phase for low values of the noise parameter. We present results for the correlation function exponent eta for several noise values. We also find that the dynamic critical exponent z is in agreement with the value expected for local update Monte Carlo rules.

Critical behavior of the two-dimensional random-bond Potts model: a short-time dynamic approach

The short-time critical dynamics of the two-dimensional eight-state random-bond Potts model is investigated with large-scale Monte Carlo simulations. Dynamic relaxation starting from a disordered and an ordered state is carefully analyzed. The continuous phase transition induced by disorder is studied, and both the dynamic and static critical exponents are estimated. The static exponent beta/nu shows little dependence on the disorder amplitude r, while the dynamic exponent z and static exponent 1/nu vary with the strength of disorder.

Corrections to scaling in two-dimensional dynamic XY and fully frustrated XY models

With large-scale Monte Carlo simulations, we investigate the two-dimensional dynamic XY and fully frustrated XY models. Dynamic relaxation starting from a disordered or an ordered state is carefully analyzed. It is confirmed that there is a logarithmic correction to scaling for a disordered start, but a power-law correction for an ordered start. Rather accurate values of the static exponent eta and the dynamic exponent z are estimated.

Universality and scaling study of the critical behavior of the two-dimensional Blume-Capel model in short-time dynamics

In this paper we study the short-time behavior of the Blume-Capel model at the tricritical point as well as along the second order critical line. Dynamic and static exponents are estimated by exploring scaling relations for the magnetization and its moments at an early stage of the dynamic evolution. Our estimates for the dynamic exponents, at the tricritical point, are z=2.215(2) and theta=-0.53(2).

Short-time critical dynamics of the two-dimensional random-bond Ising model

With Monte Carlo simulations we investigate the nonequilibrium critical dynamic behavior of the two-dimensional random-bond Ising model. Based on the short-time dynamic scaling form, we estimate all the static and dynamic exponents from dynamic processes starting with both disordered and ordered states. Corrections to scaling are carefully considered.

Monte Carlo simulations of short-time critical dynamics with a conserved quantity

With Monte Carlo simulations, we investigate short-time critical dynamics of the three-dimensional antiferromagnetic Ising model with a globally conserved magnetization m(s) (not the order parameter). From the power law behavior of the staggered magnetization (the order parameter), its second moment and the autocorrelation, we determine all static and dynamic critical exponents as well as the critical temperature. The universality class of m(s)=0 is the same as that without a conserved quantity, but the universality class of nonzero m(s) is different.

Critical dynamics of the Baxter-Wu model

The short-time behavior of the Baxter-Wu model is investigated through the relaxation of the order parameter at the critical temperature. We considered Monte Carlo simulations for this model on a triangular lattice, and we studied relaxation starting from the fourfold-degenerate ground state. Using the short-time scaling formalism we found the static critical exponents beta and nu of the model and the corresponding dynamical critical exponent z. The values of the static exponents we find agree with the exact ones. To the best of our knowledge, this is the first determination of the dynamical critical exponent of the Baxter-Wu model.

Corrections to scaling for the two-dimensional dynamic XY model

With large-scale Monte Carlo simulations, we confirm that for the two-dimensional XY model, there is a logarithmic correction to scaling in the dynamic relaxation starting from a completely disordered state, while only an inverse power law correction in the case of starting from an ordered state. The dynamic exponent z is z=2.04(1).

Dynamic approach to weak first-order phase transitions

A short-time dynamic approach to weak first-order phase transitions is proposed. Taking the two-dimensional Potts models as examples, from short-time behavior of nonequilibrium relaxational processes starting from high temperature and zero temperature states, pseudo-critical-points K* and K** are determined. A clear difference of the values for K* and K** distinguishes a weak first-order transition from a second-order one.

Short-time dynamics of an ising system on fractal structures

The short-time critical relaxation of an Ising model on a Sierpinski carpet is investigated using Monte Carlo simulation. We find that when the system is quenched from high temperature to the critical temperature, the evolution of the order parameter and its persistence probability, the susceptibility, and the autocorrelation function all show power-law scaling behavior at the short-time regime. The results suggest that the spatial heterogeneity and the fractal nature of the underlying structure do not influence the scaling behavior of the short-time critical dynamics. The critical temperature, dynamic exponent z, and other equilibrium critical exponents beta and nu of the fractal spin system are determined accurately using conventional Monte Carlo simulation algorithms. The mechanism for short-time dynamic scaling is discussed.

Out-of-equilibrium dynamics of the hopfield model in its spin-glass phase

In this paper, we study numerically the out-of-equilibrium dynamics of the Hopfield model for associative memory inside its spin-glass phase. Aside from its interest as a neural network model, it can also be considered as a prototype of a fully connected magnetic system with randomness and frustration. By adjusting the ratio between the number of stored configurations p and the total number of neurons N, one can control the phase-space structure, whose complexity can vary between the simple mean-field ferromagnet (when p=1) and that of the Sherrington Kirkpatrick spin-glass model (for a properly taken limit of an infinite number of patterns). In particular, little attention has been devoted to the spin-glass phase of this model. In this paper, we analyze the two-time autocorrelation function, the decay of the magnetization and the distribution of overlaps between states. The results show that within the spin-glass phase of the model, the dynamics exhibits aging phenomena and presents features that suggest a non trivial breaking of replica symmetry.

Critical behavior of a two-species reaction-diffusion problem

We present a Monte Carlo study in dimension d=1 of the two-species reaction-diffusion process A+B-->2B and B-->A. Below a critical value rho(c) of the conserved total density rho the system falls into an absorbing state without B particles. Above rho(c) the steady state B particle density rho(st)(B) is the order parameter. This system is related to directed percolation but in a different universality class identified by Kree et al. [Phys. Rev. A 39, 2214 (1989)]. We present an algorithm that enables us to simulate simultaneously the full range of densities rho between zero and some maximum density. From finite-size scaling we obtain the steady state exponents beta=0.435(10), nu=2.21(5), and eta=-0.606(4) for the order parameter, the correlation length, and the critical correlation function, respectively. Independent simulation indicates that the critical initial increase exponent takes the value straight theta(')=0.30(2), in agreement with the theoretical relation straight theta(')=-eta/2 due to Van Wijland et al. [Physica A 251, 179 (1998)].