Kinetic-energy pairing and condensation energy in cuprates

In order to study the condensation energy in cuprates and the possibility of kinetic-energy pairing, we study the two-dimensional Hubbard model on a square lattice near half-filling using a variational Monte Carlo method for superconductivity. We modify the variational wave function by taking account of the nearest-neighbor correlation between holons and doublons. It is found that there is a qualitative difference between the weak-coupling region and the strong-coupling region. In the strong-coupling region (U≳U_co), the energy gain at the superconducting transition is derived from the kinetic-energy, while in the weak-coupling region (U≲U_co), a conventional BCS-type potential-energy gain occurs. Condensation energy is large in the strong-coupling side and expressed as ∝exp(-4at/J) with a being a constant and J=4t²/U. This result with recent experiments of optical conductivity implies that cuprate superconductors belong to the strong-coupling region. Similar but slightly different condensation energy is also obtained in the calculation using the fluctuation exchange (FLEX) approximation.