Abstract
A method of two-time Green function for the Heisenberg model of ferromagnet of S =1/2 is developed. The equations of motion of two kinds of Green functions G (-) and G (+) , which are defined using commutator and anticommutator, respectively, are derived. Higher order Green functions are decoupled taking into account the anticommutability of Pauli operators at the same lattice site, giving coupled equations for G (-) and G (+) . The solution from G (+) are analyzed. The effect of the kinematical interaction is considered automatically, and over-all (in the entire temperature region and in the entire magnetic field region) good properties of the magnetization (which are not realized in some previous works using the first-order decoupling) are obtained. Inverse Curie temperature, J β c , is 0.398 and the critical index of the magnetization is 1/2. Several previous theories of the first-order are rederived and analyzed from a unified point of view.
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