Pairing fluctuations in a one-dimensional copper oxide model

The exact ground-state energy E0(Φ) is calculated for a one-dimensional copper-oxide model defined on a ring of finite length L, threaded by an external flux Φ. It includes on-site and nearest-neighbor intersite Coulomb repulsion V, as well as a nonzero bare ''oxide'' gap Δ0 between the orbitals in the unit cell. The correlation exponent Kρ of the corresponding low-energy Hamiltonian shows that singlet (extended s-wave) superconductivity has the most rapidly diverging susceptibility as T→0 (Kρ>1). This superconducting phase is located close to a phase-separation instability of the model, indicated by Kρ→∞. Longer range of the Coulomb interaction tends to frustrate phase separation, enhances the range of V for which Kρ>1, and slightly broadens the regime of V where 1<Kρ<∞