Optical Conductivity of HighTcSuperconductors: From Underdoped to Overdoped

We present results of room-temperature reflectivity measurements on overdoped T12201 and on underdoped and overdoped Bi2212 single crystals. The optical conductivity $\sigma \left(\omega \right)$ has two distinctly different regimes as a function of doping. While in the underdoped regime the doping-induced changes in $\sigma \left(\omega \right)$ can be well described by an increasing density of mobile carriers, in the overdoped regime there is no evidence for such an increase. Our results are not consistent with the usual view on the overdoped regime where reduction of $T_c$ is associated with an increase in the density of mobile charge carriers.