A.V. Puchkov, P. Fournier, T. Timusk,
Optical conductivity of high Tc superconductors: from
underdoped to overdoped.
We present results of room-temperature reflectivity measurements
on overdoped Tl2201 and on underdoped and overdoped Bi2212 single crystals.
The optical conductivity s(w) has two distinctly
different regimes as a function of doping. While in the underdoped regime
the doping-induced changes in s(w) 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 Tc
is associated with an increase in the density of mobile charge carriers.