eta-pairing superconductivity in the Hubbard chain with pair hopping

The ground-state phase diagram of the one-dimensional Hubbard chain with pair-hopping interaction is studied. The analysis of the model is performed using the continuum-limit field theory approach and exact diagonalization studies, At half-filling the phase diagram is shown to consist of two superconducting states with Cooper-pair center-of-mass momentum Q=0 (BCS-eta(0) phase) and Q=pi (eta(pi) phase) and four insulating phases corresponding to the Mott antiferromagnet, the Peierls dimerized phase, the charge-density-wave (CDW) insulator, and an unconventional insulating phase characterized by the coexistence of a CDW and a bond-located staggered magnetization. Away from half-filling the phase diagram consists of the superconducting BCS-eta(0) and eta(pi) phases and the metallic Luttinger-liquid phase. The BCS-eta(0) phase exhibits a smooth crossover from a weak-coupling BCS type to a strong-coupling local-pair regime. The eta(pi) phase shows the properties of the doublon (zero-size Cooper-pair) superconductor with Cooper-pair center-of-mass momentum Q = pi. The transition into the eta(pi)-paired state corresponds to an abrupt change in the ground-state structure. After the transition the conduction band is completely destroyed and a new eta(pi)-pair band, corresponding to the strongly correlated doublon motion, is created.