Crossover of Superconducting Properties and Kinetic-Energy Gain in Two-Dimensional Hubbard Model

Abstract

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∼12t from a BCS type to a new type. For U\\gtrsimU_co (U\\lesssimU_co), the energy gain in the superconducting state is derived from the kinetic (potential) energy. Condensation energy is large and ∝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.