Interplay of orbital degeneracy and superconductivity in a molecular conductor

We study electron propagation in a molecular lattice model. Each molecular site involves doubly degenerate electronic states coupled to doubly degenerate molecular vibration, leading to a so-called E-e type of Jahn-Teller Hamiltonian. For weak electron-phonon coupling and in the antiadiabatic limit we find that the orbital degeneracy induces an intersite pairing mechanism which is absent in the standard nondegenerate polaronic model. In this limit we analyze the model in the presence of an additional on-site repulsion and we determine, within BCS mean-field theory, the region of stability of superconductivity. In one dimension, where powerful analytical techniques are available, we are able to calculate the phase diagram of the model both for weak and for strong electron-phonon coupling. © 1996 The American Physical Society.