Phase separation and superconductivity in the copper-oxide chain

The phase diagram of the copper-oxide chain as a function of density and the nearest-neighbour Coulomb interaction, V, is determined. Phase separation takes place above a critical value of V when the Cu2+ → Cu+ valence fluctuations dominate. In the proximity of the phase separation boundary the superconducting correlations are the most divergent. We identify the parameter regions where the Luttinger-liquid theory applies and calculate the contours of the charge-charge correlation exponent Kρ. We show that anomalous flux quantization occurs as Kρ diverges. At n = 1, for V > t, a gap opens in the spectrum and the ground state is a charge density wave. (orig.)