The phase separation instability occurring with increasing nearest-neighbor repulsion V in a two-band Hubbard model (CuO chain) is discussed. Quantum Monte Carlo simulations indicate that this transition is associated with a level-crossing if the filling fraction is close to 1 (half-filled lower band). Spin-density-wave fluctuations then dominate before phase separation. Superconducting fluctuations dominate only at considerably higher doping levels.
@article{arxiv.cond-mat/9603076,
title = {Phase separation and the existence of superconductivity in a one-dimensional copper-oxygen model},
author = {A. W. Sandvik and A. Sudbo},
journal= {arXiv preprint arXiv:cond-mat/9603076},
year = {2009}
}