Superconductivity from electron-phonon interactions in the absence of electron-hole symmetry

We consider an electron-phonon Hamiltonian where the electron-phonon interaction occurs through a modification of both the electron on-site energy and the inter-site hopping amplitude, i.e. a combination of Holstein and Su-Schrieffer-Heeger (SSH)-like models. It is suggested that this model may apply to certain vibrational degrees of freedom in high Tc oxides. This Hamiltonian is electron-hole asymmetric and reduces in the infinite-frequency limit to a model for hole superconductivity recently discussed. We solve the Eliashberg equations for the model and examine the effect of electron-hole symmetry breaking on Tc in the presence of retardation. ((orig.))