Quasiparticle damping and the coherence peak in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

The far-infrared (30–350 ${\mathrm{cm}}^{\mathrm{-}1}$) transmittance and reflectance of two ab-plane-oriented ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films have been measured. These data show that the quasiparticle relaxation rate, 1/τ, has a fast decrease below ${\mathit{T}}_{\mathit{c}}$ and saturates at T≪${\mathit{T}}_{\mathit{c}}$. Above ${\mathit{T}}_{\mathit{c}}$, 1/τ exhibits a linear temperature dependence, in accord with the linear dc resistivity in the normal state. The fast decrease of 1/τ 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 ${\mathrm{\sigma }}_1$ 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 ${\mathrm{\sigma }}_1$ is attributed to the rapid drop in 1/τ combined with a decreasing normal-fluid density. © 1996 The American Physical Society.