A generalized coherent-potential approximation for site-disordered spin systems

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Abstract
An extension of the usual diagrammatic coherent-potential approximation is considered to account for the presence of disorder both in off-diagonal and inhomogeneous terms. The method is applied to the quenched site-disordered Ising model (S=1/2) above the transition, where the fundamental equation for the correlation function is a generalized random-phase approximation. The results for the initial slope of the critical temperature and the critical concentration are presented for the simple cubic lattice with nearest-neighbour interactions, and they are in reasonable agreement with the values derived by other methods. The differences are studied and a discussion is presented to show that, near the critical concentration, terms not considered in the coherent-potential approximation are as important as the terms included. The application of this method to other models is also briefly discussed and, in the particular case of the Heisenberg model, it is shown that the method reproduces the basic results derived by Harris et al. (1974) using an effective-medium approach.