Pairing correlations in the two-dimensional Hubbard model

We present the results of a quantum Monte Carlo study of the extended s and the d(x2-y2) pairing correlation functions for the two-dimensional Hubbard model, computed with the constrained-path Monte Carlo method. For small lattice sizes and weak interactions, we find that the d(x2-y2) pairing correlations are stronger than the extended s pairing correlations and are positive when the pair separation exceeds several lattice constants. As the system size or the interaction strength increases, the magnitude of the long-range part of both correlation functions vanishes.