Finite-Size Effects in the Two-Dimensional Negative- U Hubbard Model

Using the BCS approach and quantum Monte Carlo simulations, we investigate the size dependence of the pairing correlations in the weak and intermediate coupling regimes. Pronounced size effects appear as long as the average spacing of the single-particle energy levels is larger than or comparable to the characteristic energy scale (gap). The irregularity of the size dependence is traced to variations in the occupation of the highest nonempty energy level with system size. We also propose means to reduce the size dependence and its irregularity.