Oin the method of Yvon in crystal statistics
C. iDOMB, Bove Riley
Abstract
Closed-form approximations in crystal statistics have suffered from the defect that no steady series of approximations was available from which the rate of convergence could be assessed. The method of Yvon, based on a cluster-integral type of development, can furnish such a series of approximations. It is shown how to construct the partition functions for such approximations, the key being the use of the Mayer theory for multi-component assemblies. The method is applied to various properties of the Ising model of a ferromagnet and antiferromagnet, and results are obtained which are consistent with those based on series expansions. Previous investigations of Fournet which gave different results are shown to have made use of an insufficient number of terms in the approximation. The statistical mechanics of interacting systems on crystal lattices has attracted a good deal of attention in recent years. Exact results have been confined to a restricted class of problems in two dimensions, but for more general problems two independent approaches have been pursued. Closed-form approximations have been developed which ignore certain features of the statistical problem; and limited numbers of terms of exact series expansions for the partition function at high and low temperatures have been calculated. (See Domb (i 960) for a detailed review.) Series expansions are most useful when the terms are all consistent in sign. Convergence is then assured in the critical region, and they can yield reliable information on critical behaviour. An example of this kind is provided by the magnetic susceptibility of the Ising model above the Curie point (Domb & Sykes 1957; Fisher i959). When the terms are not consistent in sign some rearrangement must be sought, and the position is less satisfactory. For some problems, such as antiferromagnetism in a magnetic field, little information about critical behaviour can be derived from series expansions. Closed-form approximations could also be used for estimating critical behaviour if a series of successive approximations were available, so that the general trend of a particular property could be assessed. However, for most of the approximations discussed in the literature, the labour increases rapidly with the order of the approximation, and the process has not been followed for more than two or three stages. A method which does offer the possibility of a steady series of approximations,