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

Raman scattering in the heavily overdoped $(\mathrm{Y},\mathrm{Ca}){\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ ${(T}_{c}=65\mathrm{K})$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ ${(T}_{c}=55\mathrm{K})$ 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 $2{\ensuremath{\Delta}}_{0},$ ${\ensuremath{\Delta}}_{0}$ 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.