d_(x²-y²)-wave superconductivity and the Hubbard model

The numerical studies of d_{x^2-y^2}-wave pairing in the two-dimensional (2D) and the 2-leg Hubbard models are reviewed. For this purpose, the results obtained from the determinantal Quantum Monte Carlo and the density-matrix renormalization-group calculations are presented. These are calculations which were motivated by the discovery of the high-T_c cuprates. In this review, the emphasis is placed on the microscopic many-body processes which are responsible for the d_{x^2-y^2}-wave pairing correlations observed in the 2D and the 2-leg Hubbard models. In order to gain insight into these processes, the results on the effective pairing interaction as well as the magnetic, density and the single-particle excitations will be reviewed. In addition, comparisons will be made with the other numerical approaches to the Hubbard model and the numerical results on the t-J model. The results reviewed here indicate that an effective pairing interaction which is repulsive at (pi,pi) momentum transfer and enhanced single-particle spectral weight near the (pi,0) and (0,pi) points of the Brillouin zone create optimum conditions for d_{x^2-y^2}-wave pairing. These are two effects which act to enhance the d_{x^2-y^2}-wave pairing correlations in the Hubbard model. Finding additional ways is an active research problem.