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

Raman scattering in the heavily overdoped $(\mathrm{Y},\mathrm{Ca}){\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ${(T}_c=65\mathrm{}\mathrm{K})$ and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ ${(T}_c=55\mathrm{}\mathrm{K})$ 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 $2{\Delta }_0,$ ${\Delta }_0$ 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.