Variational Monte Carlo studies of gossamer superconductivity

We use a partially Gutzwiller projected BCS d-wave wave function with an antiferromagnetic-weighting factor to study the ground-state phase diagram of a half-filled Hubbard-Heisenberg model in a square lattice with nearest-neighbor hopping t and a diagonal hopping t('). The calculations are carried out by using variational Monte Carlo method which treats the Gutzwiller projection explicitly. At large on-site Coulomb interaction U, the ground state is antiferromagnetic. As U decreases, the ground state becomes superconducting and eventually metallic. The phase diagram is obtained by extensive calculations. As compared to the strong effect of U/t, the phase boundaries turn out to be less sensitive to t(')/t. The result is consistent with the phase diagram in layered organic conductors and is compared to the earlier mean-field result based on the Gutzwiller approximation.