Off-critical Casimir effect in Ising Slabs with Symmetric Boundary Conditions in d=3

Abstract

Extended de Gennes-Fisher (EdGF) local-functional method has been applied to the thermodynamic Casimir effect {\it away} from the critical point for systems in the Ising universality class confined between parallel plane plates with symmetric boundary conditions (denoted $(ab)=(++)$). Results on the universal scaling functions of the Casimir force $W_{++}(y)$ ($y$ is a temperature-dependant scaling variable) and Gibbs adsorption $\tilde G(y)$ are presented in spatial dimension $d=3$. Also, the mean-field form of the universal scaling function of the Gibbs adsorption $\tilde G(y)$ is derived within the local functional theory. Asymptotic behavior of $W_{++}(y)$ for very large values of the scaling variable $y$ is analyzed in {\it general} dimension $d$.