Structural inhomogeneities observed in YBa2Cu3O7-delta crystals with optimal transport properties

The high-T-c cuprates have exhibited a variety of unusual features in both their normal-state and superconducting properties. One of the challenges that has faced experimentalists in the field has been to determine whether the observed properties can be attributed to intrinsic behavior or whether they are a result of extrinsic factors such as sample inhomogeneity. In this paper. we present transport measurements on two YBa2Cu3O7-delta crystals which exhibit many of the features generally attributed to Fully oxygenated, homogeneous samples. Nevertheless, x-ray-diffraction studies also presented in this paper reveal surface and bulk structural inhomogeneities in both crystals. Results of these studies show variations in the c-axis lattice parameters on the surface and throughout the interiors of both crystals. These are interpreted as variations in the oxygen content. The transport studies together with the structural measurements represent a more thorough and complete characterization of sample quality than is generally reported. Our results lead one to question the validity of using transport measurements alone to determine sample quality and whether any of the properties measured in crystals can be attributed to ''optimally doped'' or ''fully oxygenated'' samples.