Infrared properties of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ and ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{Ca}}_{\mathit{n}\mathrm{-}1}$${\mathrm{Cu}}_{\mathit{n}}$${\mathrm{O}}_{2\mathit{n}+4\mathrm{}}$ thin films

We report room-temperature optical reflectivity and transmission measurements on a variety of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{Ca}}_{\mathit{n}\mathrm{-}1}$ ${\mathrm{Cu}}_{\mathit{n}}$ ${\mathrm{O}}_{2\mathit{n}+4\mathrm{}}$ thin films in the 500–25 000 ${\mathrm{cm}}^{\mathrm{-}1}$ frequency range. Reflectivity data when not biased by the substrate are used to compute the optical conductivity. A comparison of our results with published reflectivity data on single-domain ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ crystals suggests an overwhelming contribution of the ${\mathrm{CuO}}_2$ planes (with respect to the chains) to the optical conductivity. The data can be analyzed in terms of a Drude low-frequency contribution and a so-called midinfrared band. However, the anomalous infrared response appears to be uniquely described by a generalized Drude response: for all the samples investigated, the relaxation rate (which increases linearly with frequency) and the effective mass are found to be the same as a function of frequency.