Quasiparticle damping in Bi 2 Sr 2 CaCu 2 O 8 and Bi 2 Sr 2 CuO 6

The low-frequency conductivity ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}) of ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{8}$ has a Drude-like component below ${\mathit{T}}_{\mathit{c}}$. Interpreting the width of this component as the quasiparticle relaxation rate (${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$), we find that ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ decreases dramatically just below ${\mathit{T}}_{\mathit{c}}$, in sharp contrast with the T-linear ${\mathrm{\ensuremath{\tau}}}^{\mathrm{\ensuremath{-}}1}$ above ${\mathit{T}}_{\mathit{c}}$ and ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{6}$. This decrease causes a peak in ${\mathrm{\ensuremath{\sigma}}}_{1}$(\ensuremath{\omega}\ensuremath{\rightarrow}0,T) for T just below ${\mathit{T}}_{\mathit{c}}$, 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 ${\mathit{T}}_{\mathit{c}}$.