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Progress of Theoretical Physics

Concentration Expansion for Random Ising Systems

Tsuguhiro Tamaribuchi, F. Takano

Published Oct 1, 1980• 11 citations
Publisher

Abstract

Two-dimensional Ising systems with bond-impurities are treated exactly by using the method of Gibberd and Hurst. First we derive recurrence equations by which a Green's function with n scattering centers can be obtained from that with n-1 scattering centers. Using these recurrence equations, we are able to expand the free energy systematically in power series of concentration c of impure bonds. This concentration expansion is carried out up to the order of c2 for quenched and annealed systems, and we discuss critical properties of the quenched system comparing with those of the annealed system. The difference between the two systems is seen to arise first in the term of the order c2, but the singularities in these terms are almost the same for both systems. This result leads to an conjecture that critical properties of a quenched system are the same as those of an annealed system in the critical region. The shift of the critical temperature in the quenched system is also calculated up to the order of c2 in the bond-dilution case (the strength of the impurity bonds is zero).

DOI
10.1143/ptp.64.1212

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Progress of Theoretical Physics•Sep 1, 1975•17 citations
Two-dimensional Ising systems with bond-impurities are treated exactly by using the method of Gibberd and Hurst. First we derive recurrence equations by which a Green's function with n scattering centers can be obtained from that with n-1 scattering centers. Using these recurrence equations, we are able to expand the free energy systematically in power series of concentration c of impure bonds. This concentration expansion is carried out up to the order of c2 for quenched and annealed systems, and we discuss critical properties of the quenched system comparing with those of the annealed system. The difference between the two systems is seen to arise first in the term of the order c2, but the singularities in these terms are almost the same for both systems. This result leads to an conjecture that critical properties of a quenched system are the same as those of an annealed system in the critical region. The shift of the critical temperature in the quenched system is also calculated up to the order of c2 in the bond-dilution case (the strength of the impurity bonds is zero).
A new method for the concentration expansion of the free energy of the two-dimensional Ising system with binary bond mixture is given. This method is simpler and more systematic compared with that used in a previous paper by the author and Takano. By using this new method, the expansion is carried out up to the order of c3, where c is the concentration of impurity bonds. The result is shown to be consistent with the assumption on functional form of the internal energy proposed in the previous paper. This assumption leads us to a conclusion that the specific heat anomaly in the quenched system is a cusp as in the annealed system.
The two-dimensional Ising model with two kinds of bonds –ferro and antiferromagnetic– is investigated in both quenched and annealed approximations. High temperature series expansion technique is used to get the respective free energies and susceptibilities. Form the analysis of the ratio method results, the phase diagrams are drawn and critical concentrations are estimated. It is shown that in both approximations the system behaves similarly, although in the annealed case the magnetically ordered phases are more stable against increasing impurity concentration. It is also proved that for Bethe lattices initial susceptibilities for quenced and annealed systems coincide.
The techniques developed by Edwards and Anderson (1975) for the spin-glass problem are here applied to the bond-diluted quenched Ising model (S= 1 / 2 ). The n to 0 trick is used to express exactly the free energy in terms of a generalized Hamiltonian representing n different but coupled replicas. The only approximation of the theory consists of decoupling these replicas in a way that reduces the free energy of the diluted system to that of a pure system with renormalized, concentration-dependent parameters. To first order in the concentration of missing bonds the author obtains all the results previously derived by Harris (1974) using a cumulant expansion and which are exact in this limit. More generally, the theory allows a discussion of the concentration dependence of the critical temperature; in particular, the values deduced for the critical concentration agree rather well with those obtained exactly or through series expansions. The behaviour of the system near the transition is also studied and the critical exponents are found to be Fisher-renormalized.
In order to study the influence of quenched disorder on second-order phase transitions, high-temperature series expansions of the susceptibility and the free energy are obtained for the quenched bond-diluted Ising model in d=3−−5d=3--5d=3−−5 dimensions. They are analyzed using different extrapolation methods tailored to the expected singularity behaviors. In d=4d=4d=4 and 5 dimensions we confirm that the critical behavior is governed by the pure fixed point up to dilutions near the geometric bond percolation threshold. The existence and form of logarithmic corrections for the pure Ising model in d=4d=4d=4 are confirmed, and our results for the critical behavior of the diluted system are in agreement with the type of singularity predicted by renormalization group considerations. In three dimensions we find large crossover effects between the pure Ising, percolation, and random fixed points. We estimate the critical exponent of the susceptibility to be $\ensuremath{\gamma}=1.305(5)$ at the random fixed point.
Using the high-temperature series expansion method, a system of randomly mixed ferromagnetic and antiferromagnetic Ising bonds is considered in quenched and annealed systems for an arbitrary value of spin and a BCC lattice. The dependence of the critical temperature on the concentration p of ferromagnetic bonds is determined. It is found that the resulting phase diagrams depend on the system (whether quenched or annealed), the spin value and the strength of the coupling constants. Also the dependence of the critical exponent gamma on p and the crossover exponent phi are estimated.
Magnetic properties of the annealed quenched systems each in the bond and site models are investigated by the Kikuchi pair approximation. In the annealed system, phase diagrams and susceptibilities are obtained. Two critical temperatures in some case in the bond system and no critical concentration in the site system are obtained. The physical interpretation of them is given. In the quenched system, two different approximate conditions are considered for the complete randomness. The case of the first condition is equivalent to the cluster variation method proposed by Sato, Arrott and Kikuchi, and the results are in agreement with those obtained by Katsura and Matsubara. The second is essentially equivalent to the average coordination number method by Sato, Arrott and Kikuchi for the dilute ferromagnetism. However, the results for both cases are unreliable at low temperatures, because of the appearance of negative entropy. This is inevitable under the pair condition.