SURFACE TERMINATION OF YBA2CU3O7-X SYSTEMS

Owing to the complexity of their structure, YBa2Cu3O7-x systems can have several different terminations of the crystal. Even considering basal plane surfaces only, six possible terminations are found for an ideal truncation of the crystal, having different electronic properties. The characterization of the real surfaces produced by fracturing sintered materials, by cleaving a mono-crystal or by growing a thin film is a very difficult task, due to the occurrence of processes such as contaminant segregation, oxygen loss from the external planes and surface reconstruction. Here we review the experimental work on this subject and critically compare the outcomes of different spectroscopical experiments. The data are discussed in the light of the available theoretical results for the surface band structure of the ideal basal plane surfaces. The role of the surface in determining a non-superconducting surface phase is illustrated. Possible morphological and structural models for the observed surfaces are presented.