Quantum Monte Carlo study of the pairing correlation in the Hubbard ladder

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, when the enhanced pairing correlation is indeed detected if we make the energy levels of the bonding and antibonding bands lie close to each other at the Fermi level to mimic the thermodynamic limit.