Excitonic and solitonic states in one-dimensional exciton-phonon systems

A one-dimensional model for Frenkel excitons interacting with acoustical phonons via a linear diagonal coupling is studied using two distinct variational trial functions which lead to delocalized (excitonic) and localized (solitonic) states. Above a certain exciton-phonon coupling strength, which depends on the exciton bandwidth and the Debye frequency of the phonons, the soliton ground state energy lies below the continuum of dressed excitons. For exciton bandwidths not much larger than the Debye frequency this occurs in a regime where the continuum approximation for the soliton is invalid.