Doping dependence of the gap anisotropy of the high-temperature YBa2 Cu3 O7-δ superconductor

We have performed high-resolution angle-resolved photoemission spectroscopy of YBa2 Cu3 O7-δ with various hole concentrations from under-to-optimal doping. We have determined the momentum dependence of bulk superconducting gap as a function of hole concentration and found the enhancement of antinodal gap, the deviation from simple d x2 - y2 wave, and the reduction of superconducting coherence weight in the underdoped region. We also found the universality of the doping dependence of the SC-gap size and symmetry in bilayered cuprates. These results suggest the importance of the long-range pairing interactions in the underdoped region.