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Zhongguo kexue. Wulixue Lixue Tianwenxue

Long-range interaction Blume-Emery-Griffiths model: Critical exponents in microcanonical ensemble

LiangSheng LI, Ning ZHENG, QingFan SHI

Published Jul 1, 2011• 1 citation
Publisher

Abstract

We investigate the thermodynamic quantities of Blume-Emery-Griffiths model with long-range interactions and observe negative specific heat and susceptibility near the first order line, where the temperature, specific heat and susceptibility are discontinuity. When order-disorder phase transition takes place, the critical exponent of specific heat (α=α′=0) and the critical exponent of susceptibility (γ=γ′=1) are consistent with the prediction of mean field theory. However, when coupling constant corresponds to the value of canonical tri-critical point, the critical exponent of specific heat and susceptibility take the values of 0=α≠α′=1 and 1=γ≠γ′=2. The critical exponents violate the scaling homogeneity equalities and α′+β+γ′>2 breaks down the Widom scaling law.

DOI
10.1360/132010-572

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We investigate the thermodynamic quantities of Blume-Emery-Griffiths model with long-range interactions and observe negative specific heat and susceptibility near the first order line, where the temperature, specific heat and susceptibility are discontinuity. When order-disorder phase transition takes place, the critical exponent of specific heat (α=α′=0) and the critical exponent of susceptibility (γ=γ′=1) are consistent with the prediction of mean field theory. However, when coupling constant corresponds to the value of canonical tri-critical point, the critical exponent of specific heat and susceptibility take the values of 0=α≠α′=1 and 1=γ≠γ′=2. The critical exponents violate the scaling homogeneity equalities and α′+β+γ′>2 breaks down the Widom scaling law.
Abstract We explore the tricritical points and the critical lines of both Blume–Emery–Grifnths and Ising model within long-range interactions in the microcanonical ensemble. For K = K MTP , the tricritical exponents take the values β = 1/4, 1 = γ − ≠ γ + = 1/2 and 0 = α − ≠ α + = −1/2, which disagree with classical (mean held) values. When K > K MTP , the phase transition becomes second order and the critical exponents have classical values except close to the canonical tricritical parameters (K CTP ), where the values of the critical expoents become β = 1/2, 1 = γ − ≠ γ + = 2 and 0 = α − ≠ α + = 1.
Critical exponents for a one-dimensional general continuous Ising model with long-range ferromagnetic interactions decaying as 1/r1+σ are calculated using a histogram Monte Carlo technique. A continuous Ising model means that a spin can take any value between −1 and 1. The critical point behavior is investigated. It is found that the system exhibits a second-order phase transition with nonstandard critical exponents; the singularities in the specific heat and susceptibility depend on σ. For σ=1, there are extremely weak finite size effects: the first-order and the second-order cumulants of the order parameter yield ν=2.42(1). The susceptibility exponent γ=2.259(9). Results for σ=0.7 will be shown and discussed.
We introduce two variants of the Blume--Emery--Griffiths model with long-range, mean-field-like interactions and quenched disorder. Although the two variants are described by different Hamiltonians, they yield the same free energy. Moreover, both Hamiltonians lead to the same average energy and hence to the same microcanonical entropy. The canonical and microcanonical phase boundaries differ markedly, and so does the multicritical behaviour of the two ensembles. In the microcanonical ensemble, tricritical points appear for any fraction of active spins, whereas in the canonical ensemble they emerge only when this fraction is sufficiently large. At lower fractions, canonical critical endpoints arise instead. We also report the formation of gaps in the entropy as a function of magnetization, another manifestation of ensemble inequivalence.
The q-state Potts model with long-range correlated disorder is studied by means of large-scale Monte Carlo simulations for q=2, 4, 8, and 16. Evidence is given of the existence of a Griffiths phase, where the thermodynamic quantities display an algebraic finite-size scaling, in a finite range of temperatures. The critical exponents are shown to depend on both the temperature and the exponent of the algebraic decay of disorder correlations, but not on the number of states of the Potts model. The mechanism leading to the violation of hyperscaling relations is observed in the entire Griffiths phase.
The density of states of long-range Blume—Emery—Griffiths (BEG) and short-range Ising models are obtained by using Wang—Landau sampling with adaptive windows in energy and magnetization space. With accurate density of states, we are able to calculate the microcanonical specific heat of fixed magnetization introduced by Kastner et al. in the regions of positive and negative temperature. The microcanonical phase diagram of the Ising model shows a continuous phase transition at a negative temperature in energy and magnetization plane. However the phase diagram of the long-range model constructed by peaks of the microcanonical specific heat looks obviously different from the Ising chart.
The critical behaviour of the three-dimensional Blume–Emery–Griffiths (BEG) model is investigated at D / J = 0, -0.25 and -1 in the range of −1⩽ K / J ⩽0 for J = 100. The simulations are carried out on a simple cubic lattice using the heating algorithm improved from the Creutz cellular automaton (CCA) under periodic boundary conditions. The universality of the model are obtained for re-entrant and double re-entrant phase transitions which occur at certain D / J and K / J parameters, with J and K representing the nearest-neighbour bilinear and biquadratic interactions, and D being the single-ion anisotropy parameter. The values of static critical exponents β, γ and ν are estimated within the framework of the finite-size scaling theory. The results are compatible with the universal Ising critical behaviour for all continuous phase transitions in these ranges.