Thermodynamic properties of the Dicke model in the strong-coupling   regime
  We discuss the problem of a N two-level systems interacting with a single radiation mode in the strong-coupling regime. The thermodynamic properties of Dicke model are analyzed developing a perturbative expansion of the partition function in the high-temperature limit and we use this method to investigate the connections between the Dicke and the collective one-dimensional Ising model 
Giuseppe Liberti and Rosa Letizia Zaffino