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天津大学学报:英文版

用 Ising 模型计算磁性薄膜的临界指数β(L)(英文)

霍炳海, 区镜添

Published Jan 1, 1998• 0 citations
OpenAlex

Abstract

近年来出现的变分累积展开法(VCE)已被应用于统计物理中伊辛(Ising)模型的研究.本文利用VCE方法,计算了L层磁性薄膜的临界温度Tc(L)和临界指数β(L).当磁性薄膜的层数L>1时,本文的理论计算结果与实验结果大体相符.对于L=1(二维)这一特例,本文计算结果也与有关报道相同.

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近年来出现的变分累积展开法(VCE)已被应用于统计物理中伊辛(Ising)模型的研究.本文利用VCE方法,计算了L层磁性薄膜的临界温度Tc(L)和临界指数β(L).当磁性薄膜的层数L>1时,本文的理论计算结果与实验结果大体相符.对于L=1(二维)这一特例,本文计算结果也与有关报道相同.
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The variational-cumulant-expansion (VCE) method is adopted to calculate the Heisenberg model. The free energy of the model is expanded to the third order, and the analytical formulas for the critical temperature Tc{\mathit{T}}_{\mathit{c}}Tc​ are given for each order. The first order of VCE corresponds to the mean-field theory, the second order gives the same result for Tc{\mathit{T}}_{\mathit{c}}Tc​ as the 1/z expansion, and the third order gives a more complicated correction. As a comparison, the results of the two-dimensional (2D) Ising model for VCE to the seventh order are shown. We carry out a trial to predict the critical temperature of the infinite order of the VCE, and get a very accurate result for the 2D Ising model. A prediction of Tc{\mathit{T}}_{\mathit{c}}Tc​ for the Heisenberg model is also made.
It is shown for the Ising model that the linked-cluster expansion (LCE) is a special case of the variational cumulant expansion (VCE) in the high-temperature limit. The VCE was originally developed and formulated to treat lattice-gauge-field models. The free energy calculated from the VCE with a trial Hamiltonian of mean-field form in the high-temperature limit is in fact identical to that from the LCE. Thus, a more general expansion that is valid for the whole temperature range is established for the treatment of Ising systems.