SURFACE-STRUCTURE OF YBA2CU3O7-X PROBED BY REVERSED-BIAS SCANNING-TUNNELING-MICROSCOPY

We report the results of scanning tunneling microscopy studies on high-quality single crystals of YBa2Cu3O7-x (YBCO) which were cleaved along a basal plane in situ at 20 K prior to measurement. The initial results of a systematic study of the YBCO surface are presented. Reversed-bias images of the CuO chain layer reveal modulations 3.3+/-0.3\b\=1.3+/-0.1 nm in wavelength which change phase by 180-degrees under bias polarity reversal along solid chains, and are invariant under bias polarity reversal at a point defect. Regions of sharp unit-cell-sized square corrugations with disordered islands are also observed. We interpret these new results in terms of our previous model [H. L. Edwards, J. T. Markert, and A. L. de Lozanne, Phys. Rev. Lett. 69, 2967 (1992)] of the cleaved YBCO surface.