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Physical review. B, Condensed matter

Critical properties of a dilute Ising model in the percolation limit

J. Heinrichs

Published Aug 1, 1978• 1 citation
Publisher

Abstract

The critical behavior of the dilute Ising model on a Bethe lattice of coordination $z$ is studied in the percolation limit ($T\ensuremath{\rightarrow}0$) by means of a new moment-expansion procedure. Exact numerical results for the magnetization as a function of magnetic field at the critical concentration, $p={p}_{c}$, are presented for $z=3$ and $z=4$. In particular, the critical exponent ${\ensuremath{\delta}}_{P}$ is expected to agree well for very low fields with the approximate value ${\ensuremath{\delta}}_{P}=2$ obtained by Essam et al. The nature of deviations from the value ${\ensuremath{\delta}}_{P}=2$ is discussed and their consequences are noted. The case $p\ensuremath{\ne}{p}_{c}$ is also discussed.

DOI
10.1103/physrevb.18.1294

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The critical behavior of the dilute Ising model on a Bethe lattice of coordination zzz is studied in the percolation limit (T\ensuremath→0T\ensuremath{\rightarrow}0T\ensuremath→0) by means of a new moment-expansion procedure. Exact numerical results for the magnetization as a function of magnetic field at the critical concentration, p=pcp={p}_{c}p=pc​, are presented for z=3z=3z=3 and z=4z=4z=4. In particular, the critical exponent \ensuremathδP{\ensuremath{\delta}}_{P}\ensuremathδP​ is expected to agree well for very low fields with the approximate value \ensuremathδP=2{\ensuremath{\delta}}_{P}=2\ensuremathδP​=2 obtained by Essam et al. The nature of deviations from the value \ensuremathδP=2{\ensuremath{\delta}}_{P}=2\ensuremathδP​=2 is discussed and their consequences are noted. The case p\ensuremath≠pcp\ensuremath{\ne}{p}_{c}p\ensuremath=pc​ is also discussed.
Exact analytic results are presented for the critical properties of the dilute Ising model on a Bethe lattice, where a fraction p of the sites are occupied at random by spins. It is found that the critical behaviour is the same as that of the pure system (p=1) except for transitions which occur by varying p at zero temperature, where the critical behaviour is different and corresponds to the percolation problem. For T to 0 and p to p c , the percolation concentration, the zero-field magnetization shows crossover behaviour of the form expected from scaling theories. Outside the critical region, a rapidly convergent numerical method is developed and used to calculate the spontaneous magnetization.
The thermal and magnetic properties of the simple cubic Ising model with random, quenched, site impurities have been determined by Monte Carlo calculations. The transition appears to remain sharp, and the critical temperature decreases monotonically towards zero as the impurity concentration is increased towards the percolation limit. No clear evidence is found for any change in the critical exponents away from the pure lattice values. These results are discussed in terms of Harris's predictions for the critical behavior of impure systems.
We study by Monte Carlo simulations the influence of bond dilution on the three-dimensional Ising model. This paradigmatic model in its pure version displays a second-order phase transition with a positive specific heat critical exponent α\alphaα . According to the Harris criterion disorder should hence lead to a new fixed point characterized by new critical exponents. We have determined the phase diagram of the diluted model, starting from the pure model limit down to the neighbourhood of the percolation threshold. For the estimation of critical exponents, we have first performed a finite-size scaling study, where we concentrated on three different dilutions to check the stability of the disorder fixed point. We emphasize in this work the great influence of the cross-over phenomena between the pure, disorder and percolation fixed points which lead to effective critical exponents dependent on the concentration. In a second set of simulations, the temperature behaviour of physical quantities has been studied in order to characterize the disorder fixed point more accurately. In particular this allowed us to estimate ratios of some critical amplitudes. In accord with previous observations for other models this provides stronger evidence for the existence of the disorder fixed point since the amplitude ratios are more sensitive to the universality class than the critical exponents. Moreover, the question of non-self-averaging at the disorder fixed point is investigated and compared with recent results for the bond-diluted q = 4 Potts model. Overall our numerical results provide evidence that, as expected on theoretical grounds, the critical behaviour of the bond-diluted model is indeed governed by the same universality class as the site-diluted model.
The dilute A_3 model is a solvable IRF (interaction-round-a-face) model with three local states and adjacency conditions encoded by the Dynkin diagram of the Lie algebra A_3. It can be regarded as a solvable version of a critical Ising model in a magnetic field. One therefore expects the scaling limit to be governed by Zamolodchikov's integrable perturbation of the c=1/2 conformal field theory. We perform a detailed numerical investigation of the solutions of the Bethe ansatz equation for the off-critical model. Our results agree perfectly with the predicted values for the lowest masses of the stable particles and support the assumptions on the nature of the Bethe ansatz solutions which enter crucially in a recent thermodynamic Bethe ansatz calculation of the factorized scattering matrix.
We studied a new and simple method which gives successive approximate expressions for the magnetization (or any order parameter) of a random Ising model as a function of temperature, site, or bond dilution, frustration, magnetic field,... To obtain the various approximations a trace is performed on finite clusters increasing in size. In particular, we investigated a honeycomb Ising model with random nearest-neighbor interactions and calculated sequences of critical temperatures, percolation thresholds,... which converge towards exact known results.