Optical Study on Phonon Anomalies and Spin Gap in ${\mathrm{YBa}}_2({\mathrm{Cu}}_{1-\mathit{x}}{\mathrm{Zn}}_{\mathit{x}}{)}_3{\mathrm{O}}_{6.6}$

Optical reflectivity measurements with $\mathbf{E}\parallel \mathbf{c}$ on ${\mathrm{YBa}}_2({\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{)}_3{\mathrm{O}}_{6.6}$ reveal a dramatic Zn-substitution effect on the anomaly of the in-plane oxygen bending mode and the broad absorption band around $450{\mathrm{cm}}^{-1\mathrm{}}$ which are observed at low temperatures for $x=0\mathrm{}$, whereas the low- $\omega$ suppression of the optical conductivity remains unchanged. This indicates a close relation between the phonon anomaly and the absorption band, while the conductivity suppression is an independent phenomenon. The former suggests a coupling of the phonons to spin fluctuations at $(\pi ,\pi )$, leading to anomalous spectral behavior when a spin gap opens.