中文摘要：

提出一种基于自旋相干态变换求解自旋．玻色模型基态的变分法，并将其用于单原子模型，得到旋波近似和非旋波近似情形下的解析基态能量和波函数，特别是在光场与原子的弱、强耦合区域都与数值对角化结果符合得很好．另外，该方法也可以直接用于求解任意原子数的Dicke模型基态和相应的量子相变研究，而通常基于Holstein-Primakoff变换的变分法，原则上只适用于原子数趋于无穷的热力学极限情形．

英文摘要：

We present a variational method for the ground-state solutions of the spin-Bose models by means of the spin coherent-state transformation. For the Jaynes-Cummings （J-C） models with and without the rotating-wave approximation, the ground-state energies obtained by this method are in perfect agreement with the results from numerical diagonalization in the whole region of the coupling between a light field and an atom. The present variational-method can be directly used to solve the ground-state energies of the Dicke models with arbitrary atom-numbers and furoher study the quantum phase transition; while the variational-method based on the Holstein-Primakoff transformation is only valid in principle for the thermodynamic limit with the atom-number tending to infinity.