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arXiv:1107.6022 · Physical Review E

Scaling behavior of a square-lattice Ising model with competing interactions in a uniform field

S. L. A. de Queiroz

Published Sep 30, 2011• 19 citations
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Abstract

Transfer-matrix methods, with the help of finite-size scaling and conformal invariance concepts, are used to investigate the critical behavior of two-dimensional square-lattice Ising spin-1/2 systems with first- and second-neighbor interactions, both antiferromagnetic, in a uniform external field. On the critical curve separating collinearly ordered and paramagnetic phases, our estimates of the conformal anomaly c are very close to unity, indicating the presence of continuously varying exponents. This is confirmed by direct calculations, which also lend support to a weak-universality picture; however, small but consistent deviations from the Ising-like values η=1/4, γ/ν=7/4, β/ν=1/8 are found. For higher fields, on the line separating row-shifted (2×2) and disordered phases, we find values of the exponent η very close to zero.

DOI
10.1103/physreve.84.031132

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We use numerical transfer-matrix methods to investigate properties of the multicritical point of binary Ising spin glasses on a square lattice, whose location we assume to be given exactly by a conjecture advanced by Nishimori and Nemoto. We calculate the two largest Lyapunov exponents, as well as linear and nonlinear zero-field uniform susceptibilities, on strip of widths $4\ensuremath{\leqslant}L\ensuremath{\leqslant}16$ sites, from which we estimate the conformal anomaly $c$, the decay-of-correlations exponent $\ensuremath{\eta}$, and the linear and nonlinear susceptibility exponents $\ensuremath{\gamma}∕\ensuremath{\nu}$ and ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}$, with the help of finite-size scaling and conformal invariance concepts. Our results are $c=0.46(1)$; $0.187\ensuremath{\lesssim}\ensuremath{\eta}\ensuremath{\lesssim}0.196$; $\ensuremath{\gamma}∕\ensuremath{\nu}=1.797(5)$; ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}=5.59(2)$. A direct evaluation of correlation functions on the strip geometry, and of the statistics of the zeroth moment of the associated probability distribution, gives $\ensuremath{\eta}=0.194(1)$, consistent with the calculation via Lyapunov exponents. Overall, these values tend to be inconsistent with the universality class of percolation, though by small amounts. The scaling relation ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}=2\ensuremath{\gamma}∕\ensuremath{\nu}+d$ (with space dimensionality $d=2$) is obeyed to rather good accuracy, thus showing no evidence of multiscaling behavior of the susceptibilities.

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Transfer-matrix methods, with the help of finite-size scaling and conformal invariance concepts, are used to investigate the critical behavior of two-dimensional square-lattice Ising spin-1/2 systems with first- and second-neighbor interactions, both antiferromagnetic, in a uniform external field. On the critical curve separating collinearly ordered and paramagnetic phases, our estimates of the conformal anomaly c are very close to unity, indicating the presence of continuously varying exponents. This is confirmed by direct calculations, which also lend support to a weak-universality picture; however, small but consistent deviations from the Ising-like values η=1/4, γ/ν=7/4, β/ν=1/8 are found. For higher fields, on the line separating row-shifted (2×2) and disordered phases, we find values of the exponent η very close to zero.
Transfer-matrix methods are used, in conjunction with finite-size scaling and conformal invariance concepts, to generate an accurate phase diagram for a two-dimensional square-lattice Ising spin-1/2 magnet, with couplings which are positive along one coordinate axis and negative along the other, in a uniform external field. Our results indicate that the critical line starts horizontally at the zero-temperature end of the phase boundary, at variance with the re-entrant behavior predicted in some earlier studies. Estimates of the thermal scaling exponent are very close to the Ising value y(T)=1 along the critical line, except near T=0 , where strong crossover effects prevent a reliable analysis.
Using the parallel tempering algorithm and graphics processing unit accelerated techniques, we have performed large-scale Monte Carlo simulations of the Ising model on a square lattice with antiferromagnetic (repulsive) nearest-neighbor and next-nearest-neighbor interactions of the same strength and subject to a uniform magnetic field. Both transitions from the (2x1) and row-shifted (2x2) ordered phases to the paramagnetic phase are continuous. From our data analysis, re-entrance behavior of the (2x1) critical line and a bicritical point which separates the two ordered phases at T=0 are confirmed. Based on the critical exponents we obtained along the phase boundary, Suzuki's weak universality seems to hold.
The behavior of two-dimensional Ising spin glasses at the multicritical point on triangular and honeycomb lattices is investigated with the help of finite-size scaling and conformal-invariance concepts. We use transfer-matrix methods on long strips to calculate domain-wall energies, uniform susceptibilities, and spin-spin correlation functions. Accurate estimates are provided for the location of the multicritical point on both lattices, which lend strong support to a conjecture recently advanced by Takeda, Sasamoto, and Nishimori. Correlation functions are shown to obey rather strict conformal-invariance requirements, once suitable adaptations are made to account for geometric aspects of the transfer-matrix description of triangular and honeycomb lattices. The universality class of critical behavior upon crossing the ferro-para-magnetic phase boundary is probed, with the following estimates for the associated critical indices: $\ensuremath{\nu}=1.49(2)$, $\ensuremath{\gamma}=2.71(4)$, ${\ensuremath{\eta}}_{1}=0.183(3)$, which are distinctly different from the percolation values.
Abstract We study the critical properties of the antiferromagnetic spin-1/2 Ising model in an external field on the square lattice. Using tomographic entropic sampling, a flat-histogram simulation method, we estimate the number of configurations, , and related microcanonical averages in the energy-magnetization space, for system sizes L = 10–30. The critical line and exponents are calculated using finite-size scaling analysis in the temperature-external field plane. With these estimates in hand, we perform detailed studies of critical behavior using Metropolis sampling of larger systems ( ). These results are compared to several approximate theoretical methods. Our estimates of critical exponents and Binder’s reduced fourth cumulant along the critical line are in very good agreement with their respective literature values for the two-dimensional Ising universality class. We verify as well that the specific heat scales along the critical line, as expected for an Ising-like critical point.
We use numerical transfer-matrix methods to investigate properties of the multicritical point of binary Ising spin glasses on a square lattice, whose location we assume to be given exactly by a conjecture advanced by Nishimori and Nemoto. We calculate the two largest Lyapunov exponents, as well as linear and nonlinear zero-field uniform susceptibilities, on strip of widths $4\ensuremath{\leqslant}L\ensuremath{\leqslant}16$ sites, from which we estimate the conformal anomaly ccc, the decay-of-correlations exponent $\ensuremath{\eta}$, and the linear and nonlinear susceptibility exponents $\ensuremath{\gamma}∕\ensuremath{\nu}$ and ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}$, with the help of finite-size scaling and conformal invariance concepts. Our results are c=0.46(1)c=0.46(1)c=0.46(1); $0.187\ensuremath{\lesssim}\ensuremath{\eta}\ensuremath{\lesssim}0.196$; $\ensuremath{\gamma}∕\ensuremath{\nu}=1.797(5)$; ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}=5.59(2)$. A direct evaluation of correlation functions on the strip geometry, and of the statistics of the zeroth moment of the associated probability distribution, gives $\ensuremath{\eta}=0.194(1)$, consistent with the calculation via Lyapunov exponents. Overall, these values tend to be inconsistent with the universality class of percolation, though by small amounts. The scaling relation ${\ensuremath{\gamma}}^{\mathrm{nl}}∕\ensuremath{\nu}=2\ensuremath{\gamma}∕\ensuremath{\nu}+d$ (with space dimensionality d=2d=2d=2) is obeyed to rather good accuracy, thus showing no evidence of multiscaling behavior of the susceptibilities.