中文摘要：

本文研究单模光场中N个二能级原子Dicke模型的有限温度特性和相变。把原子赝自旋转换为双模费米算符，用虚时路径积分方法推导出系统的配分函数，对作用量变分求极值得到系统的热力学平衡方程，及原子布居数期待值和平均光子数随原子-光场耦合强度变化的解析表达式。重点研究了在量子涨落起主导作用的低温区，由耦合强度变化产生的从正常相到超辐射相的相变，指出该相变遵从Landau连续相变理论，平均光子数可作为序参数，零值表示正常相，大于零则为超辐射相。在零温极限下本文的结果和量子相变理论完全符合。另外，本文也讨论了系统的热力学性质，比较有限温度相变和量子相变的异同。发现，在强耦合区低温稳定态的光子数和平均能量都和绝对零度的值趋于一致，而超辐射相的熵则随耦合强度的增强迅速衰减为零。

英文摘要：

In this paper, we investigate the finite-temperature properties and phase transition of the Dicke model. Converting the atomic pseudo-spin operator to the two-mode Fermi operators, we obtain the partition function in terms of the imaginary-time path integral. The atomic population and average photon number as analytic functions of the atom-photon coupling strength are found from the thermodynamic equilibrium equation, which leads to the stationary state at a finite temperature and is determined by the variation in an extremum-condition of the Euclidean action with respect to the bosonic field. In particular we study the phase transition from normal to superradiation phase at a fixed low-temperature, in which the phase transition is dominated by quantum fluctuations. The phase transition induced by the variation of the atom-photon coupling strength indeed obeys the Landau continuous phase-transition theory, in which the average photon-number can serve as an order parameter with non-zero value that characterizes the superradiation phase. In the zero temperature limit our results recover exactly all those obtained from the quantum phase transition theory at zero temperature. In addition, we discuss the thermodynamic properties and compare the difference between finite-temperature phase transition and zero-temperature quantum phase transition. It is discovered that the average photon-number and mean energy in the low-temperature stationary state coincide with the corresponding values of zero-temperature in the strong coupling region. The entropy of the superradiation phase decays rapidly to zero with the increase of coupling strength.