OPTICAL CONDUCTIVITY OF HIGH T-C SUPERCONDUCTORS - FROM UNDERDOPED TOOVERDOPED

We present results of room-temperature reflectivity measurements on ov erdoped T12201 and on underdoped and overdoped Bi2212 single crystals. The optical conductivity sigma(omega) has two distinctly different re gimes as a function of doping. While in the underdoped regime the dopi ng-induced changes in sigma(omega) can be well described by an increas ing density of mobile carriers, in the overdoped regime there is no ev idence for such an increase. Our results are not consistent with the u sual view on the overdoped regime where reduction of T-c is associated with an increase in the density of mobile charge carriers.