Polaronic signatures in phonon isotopic shifts

The isotopic shift of c-axis O(4)-related phonon frequencies in YBa2Cu3O7 i s calculated using a model electron-phonon Hamiltonian. This Hamiltonian wa s previously used to describe the dynamics of the O(4)-Cu(1)-O(4) cluster. The model includes electronic correlations and electron-phonon interactions , exhibiting polaron tunneling. The calculated isotopic shifts of phonon ex citations differ from these found using harmonic or anharmonic potentials, and are consistent with results of optical measurements. The isotopic subst itution also changes the dynamics of polaron tunneling and produces a chang e in the local structure of the cluster.