  The dimensionality of a correlated many-body system has a large impact on its
electronic properties. When electrons are confined to one-dimensional chains of
atoms their behavior is very different than in higher dimensional systems
because they become strongly correlated, even in the case of vanishingly small
interactions. The chains consisting of copper and oxygen atoms are particularly
interesting, because the CuO orbitals are constituents of all known high
temperature superconductors. Most of previous spectroscopic studies of CuO
chain systems indicated insulating behavior[23-25]. Here we report the
discovery of momentum dependent superconducting gap and hump-peak-dip structure
in the spectra of the CuO chains. We demonstrate that superconductivity in the
chains arises due to proximity effects and the peculiar momentum dependence of
the superconducting gap shows how three dimensional coherence emerges in a
layered superconductor. The presence of the hump-dip-peak structure in the
spectra of the CuO chains is very unexpected as it was thought to only occur in
the CuO2 planes and is frequently considered to be a signature of the d-wave
pairing and the pairing boson itself.
