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 non-linear exchange between the quasi-spins leads to the increase of critical temperature as compared to traditional second order phase transition of quasi-spin system.