Properties and temperature evolution of the spectrum of localized   quasi-particles interacting with polarization phonons in two models
  Using the Feynman-Pines diagram technique, the energy spectrum of localized quasi-particles interacting with polarization phonons is calculated and analyzed in the wide range of energies at the finite temperature of the system. It is established that the general model of the system, besides the bound states known from the simplified model with an additional condition for the operator of quasi-particles number, contains the new bound states even for the systems with weak coupling. The contribution of multi-phonon processes into the formation of renormalized spectrum of the system is analyzed. The reasons of the appearance, behaviour and disappearance of separate pairs of bound states depending on the coupling constant and temperature are revealed. 
M.V. Tkach, Ju.O. Seti, O.M. Voitsekhivska, O.Yu. Pytiuk
cond-mat.str-el