POLARIZED REFLECTANCE SPECTRA OF A (001) SURFACE OF YBA2CU3O7 BEFORE AND AFTER DETWINNING

We have performed reflection measurements on a (001) surface of a YBa2Cu3O7 single crystal, using polarized light in the energy range between 50 meV and 6 eV. The crystal was grown with the typical twinned structure of this compound, showing a reflectance spectrum which was independent of the direction of polarization. The spectrum was characterized by free carrier absorption with a plasma edge near 1 eV. By applying uniaxial stress at 450°C the twinned structure was removed. The crystal surface was transformed into a single untwinned domain, resulting in a strong splitting of the plasma edge with respect to the direction of polarization. By fitting a Lorentz-Drude model to the polarized reflectance spectra of the single domain crystal, R∥a and R∥b, the tensor components of the dielectric function could be determined whereas the reflectance spectrum of the twinned surface represents an average of the components of both crystallographic directions. © 1990.