The classical two-dimensional Ising model in the static fluctuation approximation

The classical two-dimensional Ising model is studied in the framework of th e so-called static fluctuation approximation. This approximation hinges on the replacement of the square of the local-field operator with its mean val ue, The physical implication is that the true quantum-mechanical spectrum o f this operator is replaced with a distribution; the moments of this distri bution as well as the expectation value of the operator are then calculated self-consistently. With this single controllable approximation, the full t hermodynamics and the pair correlation function of the classical two-dimens ional Ising model are determined for the regular infinite lattice, The poss ible application to ultrathin ferroelectric films is discussed. (C) 1999 El sevier Science Ltd. All rights reserved.