Abstract
We report on temperature-dependent reflectance measurements on several
YBa2Cu3Ox single crystals with 6.8 ⩽ x ⩽ 7 in the far-infrared
range (5 meV < ℏω < 100 meV) with E||c. Performing a
Kramers-Kronig transformation, we derive the optical conductivity
σ||c(ℏω) and the real part of the dielectric constant 
1||c(ℏω). Fitting the data with a two-fluid model, we obtained the superfluid
carrier density and the scattering rate. The crystals of oxygen content x < 6.93, i.e., below the maximum value of Tc, show a linear
dependence of the superfluid carrier density with temperature, consistent with
the d − wave model. The data for the overdoped crystals give evidence for
superconductivity of the chain carriers.