Green's functions in the Dicke model. II. Superradiant state

The approximate equation for the Green's function of a system of emitters interacting with a photon thermal bath is investigated. A condition under which superradiance occurs in the nonequilibrium case is derived. The influence of the initial conditions on the dynamics of the system is discussed. It is shown that the kinetic equation for the total spin of the system admits (under definite conditions) an exponentially relaxing solution, the equilibrium value being obtained in the limit $t\rightarrow\infty$.