Phase transition in a system of two-level atoms interacting with an electromagnetic field

An analysis is made of the possibility of a phase transition in a system of two-level atoms interacting via a transverse electromagnetic field. The analysis is based on the total (including the polarization energy) Hamiltoman. It is shown that if allowance is made for the difference between the field acting on an atom and the Maxwellian field in the spectrum of collective (polariton) excitations, soft modes may arise and at a temperature T < T_cr a phase transition may occur and a macroscopic polarization may appear at zero frequency.