Infrared absorptivity of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ crystals

Measurements on twinned single crystals at 2 K show a smooth, approximately ${\mathrm{\omega }}^2$, absorptivity for frequencies between 80 and 400 ${\mathrm{cm}}^{\mathrm{-}1}$. Analysis of the data with a simple model conductivity suggests a constant lossy conductivity contribution in the bulk superconductor. Superimposed on the ${\mathrm{\omega }}^2$ background are absorption features corresponding to optically active phonons, plus a broad feature (ΔA≃0.2%) near ħω=3.5${\mathit{kT}}_{\mathit{c}}$. This feature is discussed in terms of possible ≃3.5${\mathit{kT}}_{\mathit{c}}$ gap excitations.