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Far-infrared reflectivity of Bi2Sr2CaCu2O8+delta single crystals with different oxygen contents

Results are reported of an infrared reflectivity study of two high Tc superconducting Bi2Sr2CaCu2O8 plus delta single crystals. One of the samples was optimally- doped (Tc equals 91 K) and the other over-doped (Tc equals 73 K). It is observed that the reflectivity of the optimally-doped crystal is near unity at low temperature (10 K) and at frequencies below 400 cm-1. The reflectivity of the over-doped crystal is significantly less than unity. The dynamical conductivity and the penetration depth of the optimally-doped crystal are similar to values for optimally-doped YBa2Cu3O7- delta single crystal. However, the over-doped crystal displays significantly larger low frequency absorption. The results can be understood within a d-wave picture in which pair breaking increases with increasing hole doping and contributes to a significant quasiparticle density below Tc.