Crossover of superconducting properties and kinetic-energy gain in two-dimensional Hubbard model

Superconductivity in the Hubbard model on a square lattice near half filling is studied using an optimization (or correlated) variational Monte Carlo method. Second-order processes of a strong-coupling expansion are considered in wave functions beyond the Gutzwiller projection. Superconductivity of d(x2-y2)-wave symmetry is widely stable, and exhibits a crossover around U = U-co similar to 12t from a BCS type to a new type. For U greater than or similar to U-co (U less than or similar to U-co), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and alpha exp(-t/J) [tiny] on the strong [weak] coupling side of U-co. With reference to experiments on optical conductivity, cuprates belong to the strong-coupling regime.