Physical Review
Critical Region for the Ising Model with a Long-Range Interaction
D. J. Thouless
Abstract
A version of the Ising model is developed in which the spin variables can be treated accurately in the continuum approximation. The perturbation series, both above and below the critical temperature ${T}_{c}$, is examined, and it is shown that there is a shift of ${T}_{c}$ from its mean-field value proportional to ${q}^{\ensuremath{-}2}\mathrm{ln}q$, as well as the well-known shift proportional to ${q}^{\ensuremath{-}1}$; here $q$ is the number of mutually interacting particles. It is shown, using renormalization theory, that there is a perturbation series in ${q}^{\ensuremath{-}1}{|T\ensuremath{-}{T}_{c}|}^{\ensuremath{-}\frac{1}{2}}$ for which all terms are finite in the limit $q\ensuremath{\rightarrow}\ensuremath{\infty}$, if the shift of ${T}_{c}$ is put in correctly. For the two-dimensional model, the shift is shown to be proportional to ${q}^{\ensuremath{-}1}\mathrm{ln}q$. Conditions are derived for a finite system to display critical behavior characteristic of three, two, one, or zero dimensions. It is shown how similar results can be obtained for a model similar to the Heisenberg model and for the standard Ising and Heisenberg models with interactions extending over many neighbors. A comparison is made between previously calculated numerical results for ${T}_{c}$ and the asymptotic forms derived here.