Anisotropy of the optical dielectric function in theabplane forYBa2Cu3O7−δ

We report ellipsometric measurements from surfaces of three single-crystal samples of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$. Two of the samples were untwinned with ${\mathit{T}}_{\mathit{c}}$=90 and 66 K. The third sample was a tetragonal single crystal with low oxygen content (δ∼0.9). We used solutions for ${\mathrm{\varepsilon }}^{\mathit{c}}$ obtained from measurements on two faces of thick, single-crystal, twinned samples to solve for the other two components of the dielectric tensor (${\mathrm{\varepsilon }}^{\mathit{a}}$ and ${\mathrm{\varepsilon }}^{\mathit{b}}$). We examine anisotropy in the ab plane and observe unexplained polarization of several peaks, in the middle range of oxygen content. Most notably, we observe a b-polarized feature at 4.16 eV and an a-polarized feature at 4.59 eV for ${\mathit{T}}_{\mathit{c}}$=66 K, which we propose to be a splitting of the 4.03-eV peak observed in the insulator.