Raman study of carrier-overdoping effects on the gap in high-T_c superconducting cuprates

Raman scattering in the heavily overdoped (Y,Ca)Ba₂Cu₃O_7-δ (T_c=65K) and Bi₂Sr₂CaCu₂O_8+δ (T_c=55K) crystals has been investigated. For both the crystals, the electronic pair-breaking peaks in the A_1g and B_1g polarizations were largely shifted to the low energies close to a half of 2Δ₀, Δ₀ being the maximum gap. It strongly suggests s-wave mixing into the d-wave superconducting order parameter and the consequent manifestation of the Coulomb screening effect in the B_1g channel. Gradual mixing of s-wave component with overdoping is not due to the change of crystal structure symmetry but a generic feature in all high-T_c superconducting cuprates.