Josephson plasma frequencies in overdoped (formula presented)

Far-infrared sphere resonance in (Formula presented) samples with various oxygen concentrations is measured down to (Formula presented) Although no peaks are observed in this frequency range for the optimumly doped sample, the Josephson plasma peak is observed at (Formula presented) for a (Formula presented)-annealed sample. The peak shifts to higher frequencies as the doping increases, and is observed at (Formula presented) for a (Formula presented)-annealed sample. The c-axis penetration depth (Formula presented) obtained from the peak frequencies is determined to be 77 to (Formula presented) These large (Formula presented) values are larger than the value estimated from the Josephson-coupled layer model, while the doping dependence is qualitatively explained by the model.