ETA-PAIRING AND SUPERCONDUCTIVITY IN THE NEGATIVE-U HUBBARD-MODEL

It is proved explicitly that the strong on-site attractive interaction acts to form eta-pairings which make full use of the phase coherence of Bloch electrons. The U(1) gauge symmetry breaks spontaneously in the formation, which is accompanied by the creation of eta-paired boson, "eta-pairon." The condensation of eta-pairons takes place at a finite temperature to make up the ground-state condensate which has off-diagonal long-range order. The resulting ground state is a singlet and the energy spectrum has a gap. This type of superconductivity based on eta-pairing exhibits the same peculiar properties as observed in high-T(c) oxide superconductors.