Doping-Dependent Superconducting Symmetry in Hubbard Systems

The d-wave superconducting state is studied by using a generalized Hubbard model, in which a next-nearest-neighbor correlated-hopping interaction (t ₃) is included. The results obtained within the BCS framework suggest the existence of a critical hole concentration, below which a d_x ^{2-y 2}-wave superconducting gap is observed. However, above this critical concentration the maxima of the single-particle excitation energy gap are rotated by /4 and no real nodes exist, as observed in tunneling experiments. In this study, the parameters estimated by first-principle calculations for cuprate superconductors are used.