Far-infrared absorptivity of single-domain ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Measurements on detwinned single crystals for radiation polarized along the a axis at 2 K give approximately 0.5% absorptivity for frequencies between ≃100 and 400 ${\mathrm{cm}}^{\mathrm{-}1}$ and a strong onset at ≃450 ${\mathrm{cm}}^{\mathrm{-}1}$. Anisotropy is observed between the a- and b-axis polarization absorptivities. The conductivity function deduced from a Kramers-Kronig analysis of the a-axis data shows low-frequency absorption bands of electronic strength.