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arXiv:cond-mat/0207007 · Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics

Solvable kinetic Gaussian model in an external field

Jian-Yang Zhu, Zhao‐Di Yang

Published Jan 1, 2000• 11 citations
arXivHTMLPDF

Abstract

In this paper, the single-spin transition dynamics is used to investigate the kinetic Gaussian model in a periodic external field. We first derive the fundamental dynamic equations, and then treat an isotropic d-dimensional hypercubic lattice Gaussian spin system with Fourier's transformation method. We obtain exactly the local magnetization and the equal-time pair-correlation function. The critical characteristics of the dynamical relaxation tau(q), the complex susceptibility chi(omega,q), and the dynamical response are discussed. The results show that the time evolution of the dynamical quantities and the dynamical responses of the system strongly depend on the frequency and the wave vector of the external field.

DOI
10.1103/physreve.61.210

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In this paper, the single-spin transition dynamics is used to investigate the kinetic Gaussian model in a periodic external field. We first derive the fundamental dynamic equations, and then treat an isotropic d-dimensional hypercubic lattice Gaussian spin system with Fourier's transformation method. We obtain exactly the local magnetization and the equal-time pair-correlation function. The critical characteristics of the dynamical relaxation tau(q), the complex susceptibility chi(omega,q), and the dynamical response are discussed. The results show that the time evolution of the dynamical quantities and the dynamical responses of the system strongly depend on the frequency and the wave vector of the external field.
We systematically investigate the dynamical response of a three-dimensional kinetic isotropic Heisenberg spin system (HSS) to an external driving field by Monte Carlo numerical simulation. The dynamical response of the isotropic HSS differs obviously from those of the anisotropic Ising spin system in which no real stable ordered state and relevant dynamical phase transition were observed in the kinetic HSS. There is a threshold time after which the magnetization m ( t ) in the HSS driven by a symmetrical external field always tends asymptotically to a disorder state regardless of the initial state of the system. The threshold time depends on the reduced temperature T / T C of the investigated spin system, amplitude H 0 and frequency of the external field, i.e. τ = C ·ω α · H 0 -β ·( T / T C ) -γ . C is a constant equal to 0.0302 for a three-dimensional lattice and exponents α = 1.18±0.01, β = 1.81±0.01 and γ = 1.68±0.01.
In this paper dynamical critical phenomena of the Gaussian model with long-range interactions decaying as on d-dimensional hypercubic lattices (d = 1, 2, and 3) are studied. First, the critical points are exactly calculated, and it is found that the critical points depend on the value of δ and the range of interactions. Then the critical dynamics is considered. We calculate the time evolutions of the local magnetizations and the spin-spin correlation functions, and further the dynamic critical exponents are obtained. For one-, two- and three-dimensional lattices, it is found that the dynamic critical exponents are all which agrees with the result when only considering nearest neighboring interactions, and that they are all δ if It shows that the dynamic critical exponents are independent of the spatial dimensionality but depend on the value of δ.
In this paper dynamical critical phenomena of the Gaussian model with long-range interactions decaying as 1/rd+δ (δ> 0) on d-dimensional hypercubic lattices (d = 1, 2, and 3) are studied. First, the critical points are exactly calculated, and it is found that the critical points depend on the value of δ and the range of interactions. Then the critical dynamics is considered. We calculate the time evolutions of the local magnetizations and the spin-spin correlation functions, and further the dynamic critical exponents are obtained. For one-, two- and three-dimensional lattices, it is found that the dynamic critical exponents are all z = 2 if δ> 2, which agrees with the result when only considering nearest neighboring interactions, and that they are all δ if 0 <δ< 2. It shows that the dynamic critical exponents are independent of the spatial dimensionality but depend on the value of δ.
In this paper, we present a multispin transition mechanism, which is an extension of the Glauber one, to investigate critical dynamics. By exactly solving the master equation, the influence of the multispin transition mechanism on the dynamic critical behavior is studied for the Gaussian model with nearest-neighbor interactions on d-dimensional lattices (d=1, 2, and 3). The time evolution of magnetization is exactly calculated, and the exact results of relaxation time and dynamic critical exponent are obtained. Our models are divided into two kinds: one is the spin-cluster transition and the other is the arbitrary multispin transition. It is found that there are different relaxation times, but the same dynamical critical exponent for different kinds of multispin transitions. The results show that the dynamical critical exponents are independent of spatial dimensions and configurations of transitional spins, and that the dynamical critical exponent is the same as that of the Glauber dynamics, and thus give a strong support to the simple single-spin-transition dynamics. Finally, we give a brief discussion on the results.
A single-spin transitional critical dynamics based on the Glauber theory is used to investigate the d-dimensional kinetic S 4 model. We derive the fundamental dynamical equations by a cumulant expansion, and then, a one-dirnensional model is treated. We obtain approximately the local magnetization and equal-time spin-pair correlation function, in which the terms of k 3 and the higher orders are ignored (corresponding to high-temperature cutoff). These are finally extended to d-dimensional case. In addition, we obtain the critical point k c of d-dimensional S 4 model, and give a relation satisfied by the dynamic critical exponent z and the correlation length critical exponent ν .
We have studied the nonequilibrium dynamic phase transitions of both three-dimensional (3D) kinetic Ising and Heisenberg spin systems in the presence of a perturbative magnetic field by Monte Carlo simulation. The feature of the phase transition is characterized by studying the distribution of the dynamical order parameter. In the case of anisotropic Ising spin system (ISS), the dynamic transition is discontinuous and continuous under low and high temperatures respectively, which indicates the existence of a tri-critical point (TCP) on the phase \linebreak boundary separating low-temperature order phase and high-temperature disorder phase. The TCP shifts towards the higher temperature region with the decrease of frequency, i.e. T TCP =1.33 × exp(−ω/30.7). In the case of the isotropic Heisenberg spin system (HSS), however, the situation on dynamic phase transition of HSS is quite different from that of ISS in that no stable dynamical phase transition was observed in kinetic HSS after a threshold time. The evolution of magnetization in the HSS driven by a symmetrical external field after a certain duration always tends asymptotically to a disorder state no matter what an initial state the system starts with. The threshold time τ depends upon the amplitude H 0 , reduced temperature T / T C and the frequency ω as τ = C .ω α . H 0 −β .( T / T C ) −γ .