Optical Study of Optimally Doped and Overdoped YBCO

Thin films of the optimally-doped and overdoped high temperature superconductor (YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ and Y$_{0.7}$Ca$_{0.3}$Ba$_{2}$Cu$_{3}$O$_{7-\delta }$ with T$_{c }$= 90 K and 79 K, respectively) have been investigated by optical spectroscopy in the ab-plane. In the normal state, with increasing the carrier concentration in the CuO$_{2}$ planes, spectral weight is lost in the high-frequency charge-transfer band and transferred to lower frequencies. With increased doping, the free-carrier (Drude-like component) plasma frequency increases, consistent with a charge density increase. However, the superfluid density decreases in this regime (overdoped region) and a substantial normal-fluid component still exists in the low frequency part of the optical conductivity well below $T_{c}$.