MEDIUM-ENERGY KIKUCHI PATTERNS FROM YBA2CU3OX(001)

The spatial distribution of quasi-elastically back-scattered electrons from single-crystalline YBa2Cu3Ox(001), a high-T-c superconductor (T- c = 91 K), has been measured at 2 keV energy with high angular resolut ion (1 degrees). The data are presented as a full-hemispherical intens ity map. It is characterized by a fourfold symmetry and resembles the Kikuchi pattern of a bcc crystal. These data are compared with single- scattering cluster simulations performed with structural models for th e near-surface layers identical to those in the bulk and for different types of the crystal termination. Rather good agreement is found when the cluster is sufficiently large (more then 700 atoms) indicating th at at room temperature the investigated near-surface layers have a loc al structure similar to the bulk one.