Investigation of the kinetic energy change in optimally-doped and overdoped YBa$_{2}$Cu$_{3}$O$_{7-\delta }$

Optimally-doped and overdoped thin films of the high temperature superconductor YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (Y-123) with T$_{c }$= 90 K and 79 K, respectively, have been investigated by infrared spectroscopy. The films were highly ab-plane-oriented. In the normal state, the charge-transfer band weight is found to decrease further in the overdoped sample. This indicates the spectral weight of charge transfer band is transferred to the mid-infrared region, just as in the case of increasing doping in the under doped regime. Below T$_{c}$, the superfluid condensation is found in both optimally-doped and overdoped samples. In the optimally-doped sample, the kinetic energy does not change significantly. However, in comparison to the optimally-doped sample, the overdoped sample shows much smaller superfluid (or condensate) density . This result indicates it is difficult to decide the kinetic energy change in the overdoped samples.