A ROOT-2X-ROOT-2 SURFACE RECONSTRUCTION OF (001) CLEAVED GDBA2CU3O7-DELTA

The atomic structure at the (001) surface of GdBa2Cu3O7-delta obtained by cleavage of melt-textured material is determined with high resolution electron microscopy. It was observed that in a majority of the clean (001) surfaces, the surface layer is a CUO2 layer neighboring the Gd-vacancy layer of the bulk (bulk-Gd-CuO2-vacuum). The preference for this termination at the CUO2 layer does not depend on delta. For this termination a square-root 2 x square-root 2 surface reconstruction occurs. For the second most occurring interruption: bulk-CuO1-delta-BaO-vacuum the surface relaxation is rather small. The surface reconstruction and surface relaxations can explain the absence of a superconducting energy gap in the photoemission spectra.