Band filling effects on coherence lengths and penetration depth in the   two-orbital negative-U Hubbard model of superconductivity
  The two-orbital superconducting state is modeled by on-site intra-orbital negative-U Hubbard correlations together with inter-orbital pair-transfer interactions. The critical temperature is mainly governed by intra-orbital attractive interactions and it can pass through an additional maximum as a function of band filling. For the certain number of electrons the clear interband proximity effect is observable in the superconducting state of the band with a smaller gap. The influence of band fillings and orbital site energies on the temperature dependencies of two-component superconductivity coherence lengths and magnetic field penetration depth is analyzed. The presence of proximity effect is probably reflected in the relative temperature behaviour of characteristic lengths. 
Grzegorz Litak, Teet \"Ord, K\"ullike R\"ago, Artjom Vargunin
cond-mat.supr-con cond-mat.str-el