Normal state of strongly coupled electrons and phonons: dielectric properties, vibrational excitations and application to high-Tc superconducting metal oxides

A new theory of many-body phenomena in a strongly coupled electron-phonon system is developed, taking into account the polaron collapse of the electron band and the instability of the phonon vacuum. A ''λ-1'' expansion and multi-phonon diagram technique are developed to obtain the dielectric response function and new vibrational excitations. Static and dynamic responses show unusual temperature, q and ω dependences. Phonons mix with polaronic plasmons thus forming new excitations - ''plasphons''. A microscopic model of the anomalous extra modes, observed in neutron scattering experiments in metal oxides is proposed. The relevance of the polaron theory of high-Tc superconductivity to high-Tc oxides is discussed in view of some recent experimental results. The charged Bose-liquid ground state (small bipolarons) is claimed to be relevant to describe some normal and superconducting state properties of metal oxides. (orig.)