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Journal of Physics C Solid State Physics

Correlations in the one-dimensional Ising model with random coupling constants

Edward R. Smith, Anthony John Guttmann

Published Jun 1, 1970• 31 citations
Publisher

Abstract

The spin-spin correlation functions of the random one-dimensional Ising model are calculated exactly and found to have a probability distribution. It is then possible to define two characteristic lengths, one to describe the behaviour of the mean and the other of the standard deviation of the probability distribution. The existence of these two characteristic lengths then suggests that the thermodynamic behaviour of the system is considerably more complex than for the corresponding nonrandom system.

DOI
10.1088/0022-3719/3/6/024

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The spin-spin correlation functions of the random one-dimensional Ising model are calculated exactly and found to have a probability distribution. It is then possible to define two characteristic lengths, one to describe the behaviour of the mean and the other of the standard deviation of the probability distribution. The existence of these two characteristic lengths then suggests that the thermodynamic behaviour of the system is considerably more complex than for the corresponding nonrandom system.
We consider the one-dimensional random-field Ising model, where the spin-spin coupling J is ferromagnetic and the external field is chosen to be +h with probability p and -h with probability 1-p. At zero temperature, we calculate an exact expression for the correlation length of the quenched average of the correlation function 〈s0{\mathit{s}}_{0}s0​sn{\mathit{s}}_{\mathit{n}}sn​〉-〈s0{\mathit{s}}_{0}s0​〉〈sn{\mathit{s}}_{\mathit{n}}sn​〉 in the case that 2J/h is not an integer. The result is a discontinuous function of 2J/h. When p=1/2, we also place a bound on the correlation length of the quenched average of the correlation function 〈s0{\mathit{s}}_{0}s0​sn{\mathit{s}}_{\mathit{n}}sn​〉.
We consider long strips of finite width L\ensuremath{\le}13 sites of ferromagnetic Ising spins with random couplings distributed according to the binary distribution P(Jij{\mathit{J}}_{\mathit{ij}}Jij​)=1/2[\ensuremath{\delta}(Jij{\mathit{J}}_{\mathit{ij}}Jij​-J0{\mathit{J}}_{0}J0​)+\ensuremath{\delta}(Jij{\mathit{J}}_{\mathit{ij}}Jij​- rJ0{\mathit{rJ}}_{0}rJ0​)], 0r1. Spin-spin correlation functions 〈${\mathrm{\ensuremath{\sigma}}}_{0}$${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$〉 along the ``infinite'' direction are computed by transfer-matrix methods, at the critical temperature of the corresponding two-dimensional system, and their probability distribution is investigated. We show that, although in-sample fluctuations do not die out as strip length is increased, averaged values converge satisfactorily. These latter are very close to the critical correlation functions of the pure Ising model, in agreement with recent Monte Carlo simulations. A scaling approach is formulated, which provides the essential aspects of the R and L dependence of the probability distribution of ln〈${\mathrm{\ensuremath{\sigma}}}_{0}$${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$〉, including the result that the appropriate scaling variable is R/L. Predictions from scaling theory are borne out by numerical data, which show the probability distribution of ln〈${\mathrm{\ensuremath{\sigma}}}_{0}$${\mathrm{\ensuremath{\sigma}}}_{\mathit{R}}$〉 to be remarkably skewed at short distances, approaching a Gaussian only as R/L\ensuremath{\gg}1. \textcopyright{} 1996 The American Physical Society.
The energy-energy correlation function of the two-dimensional Ising model with weakly fluctuating random bonds is evaluated in the large scale limit. Two correlation lengths exist in contrast to one correlation length in the pure 2D Ising model: one is finite whereas the other is divergent at the critical points. The corresponding exponent of the divergent correlation length is νe=1/2ν_e=1/2νe​=1/2 in contrast to the pure system where νe=1ν_e=1νe​=1. The calculation is based on a previously developed effective field theory for the energy density fluctuations.
An exact expression for the spin-spin correlation function is derived for the zero-temperature random-field Ising model defined on a Bethe lattice of arbitrary coordination number.The correlation length describing dynamic spinspin correlations and separated from the intrinsic topological length scale of the Bethe lattice is shown to diverge as a power law at the critical point.The critical exponents governing the behaviour of the correlation length are consistent with the mean-field values found for a hypercubic lattice with dimension greater than the upper critical dimension.
We analyse the one dimensional X-Y model with random coupling constants in a z-axis magnetic field. For the non-random system at zero temperature, there is a thermodynamic singularity at finite field, corresponding to a change in the type of spin ordering. Some thermodynamic quantities diverge and the correlation length becomes infinite. For the random system however, we can show that all the thermodynamic quantities are smoothly varying and the correlation length is bounded above at the critical point.
The two-point connected correlation function, or wavevector-dependent susceptibility, of the ferromagnetic Ising chain in a random field is calculated exactly at any temperature, for a two-parameter family of diluted symmetric exponential distributions of the magnetic fields. Thermodynamic properties of this model have been derived in a previous work by the authors. Besides the correlation function itself, the solution provides exact results for the (usual) susceptibility, the correlation length and the Edwards-Anderson parameter. The low-temperature regime is examined in full detail: the authors obtain in closed form the limit value at zero temperature of various quantities, and the first correction to this limit, which behaves linearly with temperature. The correlation length is discontinuous at zero temperature. They also derive the scaling form of the correlation function for small p, where the dilution p is the probability for a spin to be subjected to a non-zero random field. Even in this limit, the correlation function is more complicated than the simple Lorentzian predicted, e.g., by mean-field theory.