Structural perturbations at intervals of the coherence length in YBa₂Cu₃O_7−δ (δ < 0.1)

Abstract

Extremely weak structured diffuse scattering is reported in selected-area electron diffraction patterns of YBa₂Cu₃O_7−δ (δ < 0.1) and is shown to arise from static correlated displacements of atoms from their periodic lattice sites in directions approximately parallel to (28 03) and (091). This is noted to be a geometry consistent with coupled displacements of the apical oxygen and planar copper atoms over a range in the a-b plane of two unit cells. In addition, a subtle contrast, universally present in certain zone-axis high-resolution electron microscope images of YBa₂Cu₃O_7−δ (δ < 0.1), is identified and an analysis presented. It is tentatively interpreted as arising from a local perturbation of the charge density distribution along planes parallel to {449} with dimensions of less than 5Å along the (094) and (904) directions and 10–20 Å along the (110) directions. These planes intersect, partitioning successive {001} planes into cells with dimensions comparable with the zero-temperature superconducting coherence length. The oxygen pyramidal planes that link the apical oxygen atom to the two nearest oxygen atoms in the CuO₂ planes lie parallel to {449} and it is suggested that the observed structural perturbation occurs at these planes. This is consistent with displacements of apical oxygen and planar copper atoms inferred from diffraction patterns. High-resolution dark-field images are also presented which suggest that adjacent cells are very slightly misaligned, giving rise to the atomic displacements at the interfaces detected above. Further evidence that there are local deviations from the Pmmm structure (other than occasional oxygen vacancies) is shown in minor zone-axis convergent-beam electron diffraction patterns. This connected perturbation of the bonding in the copper-oxygen planes encloses regions with dimensions of the coherence length and may thus need to be considered in the interpretation of the superconducting properties of this material.