六态矢量Potts模型的相结构和相变性质

本文用变分累积展开（ＶＣＥ）方法对六态矢量Ｐｏｔｔｓ模型的相结构和相变性质进行了研究。在引进单变分参数情况下，系统的自由能，内能，比热和自发磁化强度被计算到四阶近似；在双变分参数情况下，上述物理量被计算到二阶近似。结果表明，系统存在三相结构，在较低温度下发生的是连续相变，而在较高温度下发生的是一级相变。得出的相变位置和用蒙特卡罗（ＭＣ）数值模拟方法给出的结果相一致，体现了用ＶＣＥ方法处理Ｐｏｔｔｓ模型的有效性。比用ＭＣ方法的优越性在于，用ＶＣＥ方法能体现出序参量和内能在一级相变时发生突变的情况。

The Phase Structure and Phase Transition Property on the 6-State Vector Potts Model

The phase structure and phase transition property of the 6-state vector Potts model have been studied by variational cumulant expansion method. The free energy, internal energy, specific heat and spontaneous magnetization are calculated to the 4th order approximation with the trial action of one variational parameter and to the 2nd order approximation with the trial action of two variational parameters. The result shows that there are three phases and the phase transition at lowwer temperature is continuous and at higher temperature is the first order. The positions of the phase transition points given above are in agreement with the results of Monte Carlo method.