ANISOTROPY OF THE OPTICAL DIELECTRIC FUNCTION IN THE AB PLANE FOR YBA2CU3O7-DELTA

We report ellipsometric measurements from surfaces of three single-crystal samples of YBa2Cu3O7-delta. Two of the samples were untwinned with T(c) = 90 and 66 K. The third sample was a tetragonal single crystal with low oxygen content (delta approximately 0.9). We used solutions for epsilon(c) obtained from measurements on two faces of thick, single-crystal, twinned samples to solve for the other two components of the dielectric tensor (epsilon(a) and epsilon(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 T(c) = 66 K, which we propose to be a splitting of the 4.03-eV peak observed in the insulator.