Gap anisotropy and phonon self-energy effects in single-crystalYBa2Cu3O7−δ

We report a Raman scattering investigation of superconducting gap anisotropy in single-crystal $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. Gap anisotropy is investigated by studying the peak in the low-temperature Raman continuum in various symmetries. Roughly a 35% difference in the energy of this peak is observed between different symmetries, suggesting the presence of substantial gap anisotropy. Anisotropy is further evidenced by the damping behavior of the 340-${\mathrm{cm}}^{-1\mathrm{}}$ Raman-active phonon below $T_c$, which displays increased attenuation due to phonon-induced pair breaking. The temperature dependence of this attenuation is consistent with a $T=0\mathrm{}$ pair-breaking peak which is much larger than the 340-${\mathrm{cm}}^{-1\mathrm{}}$ phonon in certain regions of the Fermi surface.