Infrared reflectivity of untwinned ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$${)}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals

The ab-plane reflectivity of untwinned ${\mathrm{YBa}}_2$(${\mathrm{Cu}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$ ${)}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals is measured between 250 and 20 000 ${\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. Through Zn substitution, the optical conductivity along E∥b shows a broad peak at about 2000 ${\mathrm{cm}}^{\mathrm{-}1}$, suggesting localization of the chain band carrier. The conductivity along E∥a shows no mid-infrared absorption and slightly decreases from the far- to mid-infrared region through Zn substitution. Analysis of the conductivity along E∥a shows that the main effect of Zn substitution is to increase the impurity scattering rate. A small amount of carrier reduction and suppression of effective mass enhancement at low frequency are also observed. The suppression of the effective mass enhancement is discussed by the antiferromagnetic spin-fluctuation theory of Fermi liquid.