STRUCTURAL PERTURBATIONS AT INTERVALS OF THE COHERENCE LENGTH IN YBA2CU3O7-DELTA (DELTA-LESS-THAN-0.1)

Extremely weak structured diffuse scattering is reported in selected-a rea electron diffraction patterns of YBa2Cu3O7-delta (delta<0.1) and i s shown to arise from static correlated displacements of atoms from th eir periodic lattice sites in directions approximately parallel to [28 0 3] and [091]. This is noted to be a geometry consistent with couple d displacements of the apical oxygen and planar copper atoms over a ra nge in the a-b plane of two unit cells. In addition, a subtle contrast , universally present in certain zone-axis high-resolution electron mi croscope images of YBa2Cu3O7-delta (delta<0.1), is identified and an a nalysis presented. It is tentatively interpreted as arising from a loc al perturbation of the charge density distribution along planes parall el to {449} with dimensions of less than 5 Angstrom along the [094] an d [904] directions and 10-20 Angstrom along the [110] directions. Thes e planes intersect, partitioning successive {001} planes into cells wi th dimensions comparable with the zero-temperature superconducting coh erence 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 apic al oxygen and planar copper atoms inferred from diffraction patterns. High-resolution dark-field images are also presented which suggest tha t adjacent cells are very slightly misaligned, giving rise to the atom ic displacements at the interfaces detected above. Further evidence th at there are local deviations from the Pmmm structure (other than occa sional oxygen vacancies) is shown in minor zone-axis convergent-beam e lectron diffraction patterns. This connected perturbation of the bondi ng in the copper-oxygen planes encloses regions with dimensions of the coherence length and may thus need to be considered in the interpreta tion of the superconducting properties of this material.