A method of moments applied to the diluted ferromagnet

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Abstract
An approximation method for determining the response function of a system given the first few frequency moments is presented. The response is assumed to be well represented by poles in the second unphysical sheet of the complex cut frequency plane. Technically, such a description is achieved by first introducing a nonlinear transformation of the frequency to map both first and second sheets on to a single plane of a new variable. The moment series in the new variable is then represented by a Pade approximant. The technique is illustrated by application to a model of a diluted ferromagnet for which the first ten frequency moments of the zero- temperature response have been calculated for arbitrary wavevector and arbitrary concentration of nonmagnetic impurities. Aspects of the diluted ferromagnet that cannot be adequately treated in an effective-medium approach are discussed.

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