C. Buzano et al., A CLUSTER VARIATION APPROACH TO THE RANDOM-ANISOTROPY BLUME-EMERY-GRIFFITHS MODEL, Journal of physics. Condensed matter, 6(2), 1994, pp. 327-338
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.