Temperature dependence of the linewidths of the Raman-active phonons ofYBa2Cu3O7: Evidence for a superconducting gap between 440 and 500cm−1

The linewidths and frequencies of three Raman-active phonons have been measured in twin-free single crystals of ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. Below ${\mathit{T}}_{\mathit{c}}$, the phonon at 340 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ sharpens dramatically, the phonon at 440 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ sharpens slightly, and the phonon at 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ broadens. The temperature dependence of the phonon linewidths places the superconducting energy gap (2\ensuremath{\Delta}) between 440 and 500 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, or 2\ensuremath{\Delta}=6.8${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$--7.7${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$. Based upon the observed change in linewidth, the electron-phonon coupling strength of the 340-${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ phonon is estimated to be between 0.6 and 0.8.