A Second Order Green Function Theory of a Heisenberg Ferromagnet

概要

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Magnetic and thermodynamic properties in the paramagnetic phase of a Heisenberg ferromagnet are investigated with the use of double-time Green functions. A slightly modified decoupling scheme is presented in the second stage of the equations of motion on Green functions. The present results obtained under the assumiption of the isotropy give somewhat lower critical temperatures for various spin values than those by the high-temperature extrapolation techniques, and show that there is no long-range order at finite temperatures for a two-dimensional square lattice. In the case S=∞ or the case in the high-temperature limit our results agree with those by the RPA Green function theory. The nearest neighboring spin-pair correlations for fcc lattice at the critical temperature are in excellent agreement with those of Domb and Sykes:c_1/c_0=0.191,0.229,0.256(present theory);c_1/c_0=0.199,0.224,0.252(Domb and Sykes)for S=1/2, 1, ∞, respectively, in each case. The asymptotic behavior of the magnetic and the thermodynamic properties just above the critical temperature reveals the similar temperature-dependence as in the RPA Green function theory.
理論物理学刊行会の論文
1973-11-25