Topography of YBa2Cu3O7-δ single crystals, spectroscopy of thin films and sintered YBa2Cu3O7-δ: Theory and STM observations

Abstract

We report topographic scanning tunnelling microscopy measurements on the (001) and (100) surfaces of YBa2Cu3O7-δ single crystals, together with a theoretical basis by which the surface structures can be understood. We show that it is possible to obtain reproducible topographic data on a clean (001) and (100) surface in air. The results show various structures at scales from 0·1 to 100 nm. Among the observed features on the (001) surface were growth steps which had heights that were multiples of the lattice constant in the [001] direction. This indicates that the bulk crystal structure extends to the surface, and that the surface is relatively stable, even in humid air. We also found small (013) facets on the (001) surface, in accordance with theoretical pradictions. Occasionally, we found evidence for a one-dimensional corrugation on the surface. This is possibly a direct consequence of the orthorhombic distortion in the a-b plane with ordered oxygen vacancies, which gives rise to chain-like Cu-O structures along the b-axis. In addition, we measured the energy gap of sintered pellets and thin films of this superconductor, using a low-temperature STM. We discuss some effects which may influence the measurements and present criteria to check for these effects. We found an energy gap of δ=14±2 eV, in good agreement with the BCS pradiction 2δ/kTc=3·5. This is not contradicted by the results we obtained by far infrared spectroscopy and Andreev reflection measurements.