Isotopic substitution in a model polaronic system

The effect of isotopic substitution in the excitation spectrum of a model p olaronic system is presented. To study this effect we use a three-site many -body Hamiltonian, with two phonon modes, which includes electronic correla tions and electron-phonon interactions. Under isotopic substitution, the sh ift of the lowest excitation energy changes sign for intermediate electron- phonon coupling, coinciding with the onset of polaronic behavior. This mani fests as a strong change of the dynamics of polaron tunneling, a behavior o pposite to that found in purely phonon excitations. Additionally, a small c hange in the static polaronic distortion appears under isotopic substitutio n. This change in the local structure has opposite signs for low and interm ediate electron-phonon coupling compared to that found in the strong-coupli ng limit. [S0163-1829(99)08013-3].