Model of resonant interaction between long-wavelength photons and two-level atoms in solids

A modification of the Dicke model is proposed in which allowance is made for direct interaction between atoms polarized by the electromagnetic field and their thermal vibrations in a crystal. It is shown that depending on the parameters, both second- and first-order phase transitions are possible in this system. Rigorous equations are obtained for the order parameter of the atomic subsystem, the difference between the level populations, and the collective precession of the dipole moments.