  We study superconductivity in two-band models where one of the bands does or
does not intersect the Fermi level depending on the parameter values. Applying
a many-variable variational Monte-Carlo method for a Hubbard model on two-leg
ladder and bilayer square lattices, we show that superconductivity can be
enhanced in a parameter regime where the edge of one of the bands is near the
Fermi energy, that is, when the band is incipient. The resemblence of the
present results to those obtained by a weak coupling method in a recent study
suggests that, even in the large $U$ regime, the suppression of the
near-zero-energy spin fluctuations and the development of finite energy spin
fluctuations are the key factors for the enhancement of superconductivity by an
incipient band.
