Doping dependence of the superconducting gap in Bi₂Sr₂CaCu₂O_8+δ

Bi₂Sr₂CaCu₂O_8+δ crystals with varying hole concentrations (0.12<p<0.23) were studied to investigate the effects of doping on the symmetry and magnitude of the superconducting gap [Δ(k)]. Electronic Raman scattering experiments that sample regions of the Fermi surface near the diagonal (B_2g) and principal axes (B_1g) of the Brillouin zone have been utilized. The frequency dependence of the Raman response function χ^″ at low energies [ω<Δ(k)] is found to be linear for B_2g and cubic for B_1g (T<T_c). The latter observations have led us to conclude that the doping dependence of the superconducting gap is consistent with d_x²-y² symmetry, for slightly underdoped and overdoped crystals. Studies of the pair-breaking peak found in the B_1g spectra demonstrate that the magnitude of the maximum gap (|Δ_max|) decreases monotonically with increasing hole doping, for p>0.12. Based on the magnitude of the B_1g renormalization, it is found that the number of quasiparticles participating in pairing increases monotonically with increased doping. On the other hand, the B_2g spectra show a weak “pair-breaking peak” that follows a paraboliclike dependence on hole concentration, for 0.12<p<0.23.