Determination of the superconducting gap in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The anomalous broadening of the ${\mathit{B}}_{1\mathit{g}}$-like phonon of R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ observed in Raman scattering is studied as a function of phonon frequency by substituting various rare earths for R. The onset of broadening starts abruptly at a phonon energy which allows us to unambiguously determine a single, sharp superconducting gap at 2Δ=316 ${\mathrm{cm}}^{\mathrm{-}1}$. 2Δ/${\mathit{kT}}_{\mathit{c}}$=4.95±0.10, i.e., R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ is a strong-coupling superconductor. We find qualitative and quantitative agreement with theory.