FIGURE 4. Schematic illustrations of the gap function evolution for three different doping levels.

a, Underdoped sample with T_c = 75 K. b, Underdoped sample with T_c = 92 K. c, Overdoped sample with T_c = 86 K. At 10 K above T_c there exists a gapless Fermi arc region near the node; a pseudogap has already fully developed near the antinodal region (red curves). With increasing doping, this gapless Fermi arc elongates (thick red curve on the Fermi surface), as the pseudogap effect weakens. At T < T_c a d-wave like superconducting gap begins to open near the nodal region (green curves); however, the gap profile in the antinodal region deviates from the simple d_x²_{- y}² form. At a temperature well below T_c (T  T_c) the superconducting gap with the simple d_x²_{- y}² form eventually extends across entire Fermi surface (blue curves) in b and c but not in a.