Specificity of Phase Transition of Quasi-Spin System in Two-Quantum   Exchange with Thermostat
  The quantum phase transition of the system of N radiators in case of two-photon exchange interaction with cavity electromagnetic field is considered. It is shown that for this system the atom-field exchange integral increases with the increase of temperature. This effect generates peculiarities in phase transition picture. The process of nonlinear exchange between the quasi-spins leads to the increase of critical temperature as compared to traditional second order phase transition of quasi-spin system. 
Nicolae Enaki, Vitalie Eremeev