Isotopic substitution in a model polaronic system

The effect of isotopic substitution in the excitation spectrum of a model polaronic system is presented. To study this effect we use a three-site many-body Hamiltonian, with two phonon modes, which includes electronic correlations and electron-phonon interactions. Under isotopic substitution, the shift of the lowest excitation energy changes sign for intermediate electron-phonon coupling, coinciding with the onset of polaronic behavior. This manifests as a strong change of the dynamics of polaron tunneling, a behavior opposite to that found in purely phonon excitations. Additionally, a small change in the static polaronic distortion appears under isotopic substitution. This change in the local structure has opposite signs for low and intermediate electron-phonon coupling compared to that found in the strong-coupling limit.