A CLUSTER VARIATION APPROACH TO THE RANDOM-ANISOTROPY BLUME-EMERY-GRIFFITHS MODEL

The random-anisotropy Blume-Emery-Griffiths model, which has been prop osed to describe the critical behaviour of He-3-He-4 mixtures in a por ous medium, is studied in the pair approximation of the cluster variat ion method extended to disordered systems. Several new features, with respect to mean-field theory, are found, including a rich ground state , a non-zero percolation threshold, a reentrant coexistence curve and a miscibility gap on the high-He-3-concentration side down to zero tem perature. Furthermore, nearest-neighbour correlations are introduced i nto the random distribution of the anisotropy, and are shown to be res ponsible for the raising of the critical temperature with respect to t he pure and uncorrelated random cases and to contribute to the detachm ent of the coexistence curve from the lambda line.