海森伯铁磁体的热力学微扰理论——Ⅰ.基本理论Ⅱ.双时格林函数的切断近似

A Thermodynamic Perturbation Theory of Heisenberg Ferromangnets Ⅰ. Fundamental Theory Ⅱ. Decoupling Approximations in Green's Function Method

Research Laboratory of Microelectronics, Department of Radio Sciences and Electronics

摘要: 本文建立了海森伯铁磁体的热力学微扰理论,它在自旋算符的运动方程中分出了微扰部分,格林函数的微扰展式在经过适当的"重整化"以后与费密子体系的形式相同,从而可以把Feynman图解方法应用于自旋体系。这个微扰理论的特点是能将运动学相互作用与动力学相互作用分离。
在本文的后一部分,利用上述微扰理论分析和澄清了双时格林函数的各种切断近似,并在此基础上提出了一种新的切断近似,它能得出磁化强度的正确低温展式。

Abstract: A thertmodynamic perturbation theory of the Heisenberg ferromagneis is developed, which identifies a perturbation part in the motion equation of the spin operator. After appropriate "renormalization" the perturbation expansion of the Green's function has the same form as that for fermion systems. Thus the Feynman diagrammatic technique can be extended to spin systems. This theory has the distinguishing feature of separating the kinetic interactions from the dynamic interactions.
Then this theory is applied to analyzing various decoupling approximations in Green's function method. And a new decoupling procedure is proposed, which can give the low temperature expansion of magnetization correct up to the T term.