FAR-INFRARED STUDIES OF RESIDUAL UNPAIRED CARRIERS IN YBA2CU3O7-DELTA CRYSTALS

Optical studies of the c-axis response of highly oxygenated YBa2Cu3O7-delta crystals are presented. To characterize our crystals we have performed Raman measurements at room temperature. We found no disorder induced phonon modes and an extremely high frequency of the apical oxygen phonon mode (up till approximate to 517 cm(-1)). The c-axis reflectivity is characterized by a metallic frequency and temperature dependence. The corresponding low frequency (omega-->0) conductivity strongly increases with reducing temperature, which is in agreement with de transport measurements. At temperatures far below T-c we observe finite absorptivity for frequencies omega<2 Delta/h giving rise to an increase of the conductivity towards small frequencies, which we attribute to the presence of residual unpaired carriers. We find that the residual normal carrier density is approximately 70% of the missing area in the low-temperature conductivity.