Quasiparticle damping and the coherence peak in YBa2Cu3O7-delta

The far-infrared (30-350 cm(-1)) transmittance and reflectance of two ab-plane-oriented YBa2Cu3O7-delta films have been measured. These data show that the quasiparticle relaxation rate, 1/tau; has a fast decrease below T-c and saturates at T much less than T-c. Above T-c, 1/tau exhibits a linear temperature dependence, in accord with the linear dc resistivity in the normal state. The fast decrease of 1/tau is unique and intrinsic to the high-T-c cuprates; it does not occur in conventional BCS superconductors, where the scattering is from impurity or phonons. The !Ow-frequency conductivity sigma(1) inferred from the far-infrared transmittance and reflectance measurements exhibits a peak just below T-c. The conductivity peak occurs at higher frequencies than those expected for BCS coherence effects. The peak in sigma(1) is attributed to the rapid drop in 1/tau confined with a decreasing normal-fluid density.