Superconductivity and spin gap in the high-doping regime of a CuO chain model

A two-band Hubbard model describing a CuO chain is studied using quantum Monte Carlo simulation. The Luttinger-liquid exponent Kρ is extracted as a function of the nearest-neighbor repulsion V, at hole fillings left angle n right angle between 1 (half-filled lower band) and 2. Evidence is presented for the existence of a regime with dominant singlet superconducting fluctuations and a gap in the spin excitation spectrum at left angle n right angle = 1.75, in the neighborhood of the V for which the system phase separates. Closer to half-filling (left angle n right angle <or∼ 1.6), spin-density-wave fluctuations always dominate. (orig.)