Ising Model and Bethe Approximation in Itinerant-Electron Systems. I

Abstract

It is shown that the decoupling approximation in functional integral method, which has been used by Hasegawa, leads to a generalized Ising model in itinerant-electron systems, and then a new theory to go beyond the single-site approximation is proposed. It is based on the Bethe approximation in Ising model. Present theory gives SRO of 0.1∼0.2 for Fe, Co and Ni at T∼T_c.