  An extensive Quantum Monte Carlo calculation is performed for the two-leg
Hubbard ladder model to clarify whether the singlet pairing correlation decays
slowly, which is predicted from the weak-coupling theory but controversial from
numerical studies. Our result suggests that the discreteness of energy levels
in finite systems affects the correlation enormously, where the enhanced
pairing correlation is indeed detected if we make the energy levels of the
bonding and anti-bonding bands lie close to each other at the Fermi level to
mimic the thermodynamic limit.

Strongly Correlated Electrons