Quasiparticle damping and the coherence peak in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mi mathvariant="normal">−</mml:mi><mml:mi mathvariant="normal">δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>

The far-infrared (30--350 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) transmittance and reflectance of two ab-plane-oriented ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ films have been measured. These data show that the quasiparticle relaxation rate, 1/\ensuremath{\tau}, has a fast decrease below ${\mathit{T}}_{\mathit{c}}$ and saturates at T\ensuremath{\ll}${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$, 1/\ensuremath{\tau} exhibits a linear temperature dependence, in accord with the linear dc resistivity in the normal state. The fast decrease of 1/\ensuremath{\tau} is unique and intrinsic to the high-${\mathit{T}}_{\mathit{c}}$ cuprates; it does not occur in conventional BCS superconductors, where the scattering is from impurity or phonons. The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$ inferred from the far-infrared transmittance and reflectance measurements exhibits a peak just below ${\mathit{T}}_{\mathit{c}}$. The conductivity peak occurs at higher frequencies than those expected for BCS coherence effects. The peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$ is attributed to the rapid drop in 1/\ensuremath{\tau} combined with a decreasing normal-fluid density. \textcopyright{} 1996 The American Physical Society.