  A variational Monte Carlo calculation is used for studying the ground state
of the two-dimensional Hubbard model, including hopping between both nearest
and next-nearest neighbor sites. Superconductivity with d-wave symmetry is
found to be restricted to densities where the Fermi surface crosses the
magnetic zone boundary. The condensate energy is much larger for hole doping
than for electron doping. Superconductivity is kinetic energy driven for hole
doping, but potential energy driven for electron doping. Our findings agree
surprisingly well with experimental data for layered cuprates, both for
electron- and hole-doped materials.
