Coherence lengths for superconductivity in the two-orbital negative-U   Hubbard model
  We study the peculiarities of coherency in the superconductivity of two-orbital system. The superconducting phase transition is caused here by the on-site intra-orbital attractions (negative-U Hubbard model) and inter-orbital pair-transfer interaction. The dependencies of critical and noncritical correlation lengths on interaction channels and band fillings are analyzed. 
G. Litak, T. Ord, K. Rago, and A. Vargunin
cond-mat.supr-con cond-mat.str-el