Quasiparticle damping in Bi2Sr2CaCu2O8 and Bi2Sr2CuO6

The low-frequency conductivity 1() of Bi2Sr2CaCu2O8 has a Drude-like component below Tc. Interpreting the width of this component as the quasiparticle relaxation rate , we find that decreases dramatically just below Tc, in sharp contrast with the T-linear above Tc and Bi2Sr2CuO6. This decrease causes a peak in 1 for T just below Tc, 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 Tc.