Raman scattering from superconducting gap excitations in single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The results of a polarized light scattering study of single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are presented. Strong interband electronic scattering in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is observed through interference effects with certain phonons. Below $T_c$, this electronic scattering undergoes a redistribution due to gap formation, which is monitored by a rapid change in the damping rates of the strongly coupled phonons. Our lowest temperature data further indicates the presence of residual electronic scattering well below the expected 2Δ gap onset, suggesting that a continuum of electronic states exists inside the gap.