Using polarized reflectivity measurements of single-domain crystals, we are able to distinguish chain and plane contributions to the infrared conductivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$. A substantial chain contribution to σ(ω) persisting to low frequency and temperature is observed. For the intrinsic conductivity of the ${\mathrm{CuO}}_2$ planes a superconducting energy gap of 500 ${\mathrm{cm}}^{\mathrm{-}1}$ (2Δ/${\mathit{kT}}_{\mathit{c}}$≃8) is evident in the infrared data, while the normal-state conductivity drops much more slowly with ω than the ordinary Drude form, and can be described in terms of a scattering rate ħ/${\mathrm{\tau }}^{\mathrm{*}}$∼kT+ħω at low frequency. The former result (2Δ/${\mathit{kT}}_{\mathit{c}}$≃8) suggests suppression of ${\mathit{T}}_{\mathit{c}}$; the latter, that ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ is not an ordinary Fermi liquid.

• Received 19 March 1990

DOI:https://doi.org/10.1103/PhysRevLett.65.801