REALIZATION OF NEGATIVE-U SUPERCONDUCTIVITY IN A CLASS OF PURELY REPULSIVE SYSTEMS - INTERACTING CARRIER AND INSULATING BANDS

The occurrence of superconductivity due to nonretarded attraction is demonstrated in a class of repulsively interacting electron systems that consist of a carrier band interacting with an insulating band. The superconductivity, which can be mapped to that in the attractive Hubbard model, has been confirmed from both the canonical transformation of the Hamiltonian and quantum Monte Carlo results. We indicate essential differences of the present models from existing models, including the d-p(sigma) model, and also discuss the relevance of our models to high-T(c) materials.