ANISOTROPY AND OXYGEN-STOICHIOMETRY DEPENDENCE OF THE DIELECTRIC TENSOR OF YBA2CU3O7-DELTA (0 LESS-THAN-OR-EQUAL-TO DELTA LESS-THAN-OR-EQUAL-TO 1)

The dielectric tensor elements of oriented YBa2Cu3O7-delta samples, as obtained by spectroscopic ellipsometry, are presented for several oxygen stoichiometries in the photon energy range between 1.66 and 5.5 eV. For delta = 0 and 1, we extend this range up to 9.5 eV using a recently developed synchrotron radiation ellipsometer. We compare the experimental results with computed values for the dielectric function obtained from a linear-muffin-tin-orbitals band-structure calculation. We find good general agreement for both the orientational and compositional dependence. In particular, both experiment and theory show c polarization for a peak near 2.7 eV for delta < 0. 5, while a transition in that spectral region is a polarized for YBa2Cu3O6. In Spite of the difficulty in predicting insulating behavior of the low-oxygen material, these results show that the local-density-approximation picture is successful in describing much of the electronic structure determining higher-energy excitations. In addition, the measured dependence of the dielectric tensor on the oxygen stoichiometry provides a reference for optical determination of the oxygen content and orientation in applications, such as thin-film or single-crystal growth.