Abrupt Transition in Quasiparticle Dynamics at Optimal Doping in a Cuprate Superconductor System

We report time-resolved measurements of the photoinduced change in reflectivity, $\Delta R$, in the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}}_{1-y}{\mathrm{Dy}}_y{\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ (BSCCO) system of cuprate superconductors as a function of hole concentration. We find that the kinetics of quasiparticle decay and the sign of $\Delta R$ both change abruptly where the superconducting transition temperature $T_c$ is maximal. These coincident changes suggest that a sharp transition in quasiparticle dynamics takes place precisely at optimal doping in the BSCCO system.