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° 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.