Mid-infrared reflectivity and ellipsometry measurements on single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_{6+\mathrm{y}}$

We have measured the room-temperature reflectivity of ‘‘90-K’’ single crystals ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ in the frequency range 600–9000 ${\mathrm{cm}}^{\mathrm{-}1}$. The reflectivity R in these highly conducting samples (${\rho }_{\mathrm{ab}}$=150 μΩ cm at 290 K) is found to be higher than in previous reports. We fit R to a simple Drude-Lorentz model and compare the fit parameters with the dc transport values. (The effective-mass ratio of the carriers is found to be 2.0, and the scattering rate ħ/τ is 3.1$k_B$T at room temperature.) Ellipsometry measurements have also been performed using transverse-electric and transverse-magnetic polarizations. The dielectric dispersion derived from ellipsometry shows some important deviations from the Drude-Lorentz model. Reflectivity data from nonsuperconducting crystals of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_{6+\mathrm{y}}$ are also reported.