QUASI-PARTICLE DAMPING IN BI2SR2CACU2O8 AND BI2SR2CUO6

The low-frequency conductivity sigma-1(omega) of Bi2Sr2CaCu2O8 has a Drude-like component below T(c). Interpreting the width of this component as the quasiparticle relaxation rate (tau--1), we find that tau--1 decreases dramatically just below T(c), in sharp contrast with the T-linear tau--1 above T(c) and in Bi2Sr2CuO6. This decrease causes a peak in sigma-1(omega --> 0, T) for T just below T(c), a peak which is due to the scattering rate and not to pair coherence effects, consistent with the lack of a coherence peak in the NMR relaxation rate. This result implies that the excitations which scatter the carriers are suppressed below T(c).