One-dimensional electron-phonon system at arbitrary coupling constant

The ground state of a quasi-particle (exciton, electron or hole) interacting with optical and acoustic phonons in a one-dimensional chain, is investigated using a variational method. The phase diagrams are obtained which show the regions of the electron-phonon coupling constant and nonadiabaticity parameter where the ground state of a quasiparticle is described as almost free electron state, small polaron, or spontaneously localized state. It is shown that the formation of a soliton-like state has a threshold with respect to the value of electron-phonon interaction. The corresponding critical value of the coupling constant increases with the nonadiabaticity parameter increasing.