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 the strong-coupling expansion are considered in the wave functions beyond the Gutzwiller factor. Superconductivity of dx2-y2-wave is widely stable, and exhibits a crossover around U=Uco 12t from a BCS type to a new type. For U Uco (U Uco), 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 Uco. Cuprates belong to the strong-coupling regime.
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