It is proved explicitly that the strong on-site attractive interaction acts to form η 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 η-paired boson, ‘‘η pairon.’’ The condensation of η 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 η pairing exhibits the same peculiar properties as observed in high-${\mathit{T}}_{\mathit{c}}$ oxide superconductors.

• Received 30 November 1990

DOI:https://doi.org/10.1103/PhysRevB.44.871