ORIGIN OF LOW-TEMPERATURE RESIDUAL ABSORPTION IN YBA2CU3OY

We have measured the c axis reflectivity of over- and slightly underdoped YBa2Cu3Oy crystals. It was found that an additional free carrier-like contribution, observed in the low-temperature (T < T-c) conductivity of the fully oxygenated crystals, is characteristic for the overdoped state and disappears in the underdoped regime. The decrease of the penetration depth lambda(c) with increasing hole doping saturates in the overdoped regime. The related saturation of the superfluid density is compensated by an enhanced quasi particle density that is attributed to an intrinsic pair-breaking process.