SCANNING TUNNELING MICROSCOPIC STUDIES OF GROWTH SPIRALS IN BULK SINGLE-CRYSTALS OF SUPERCONDUCTING YBA2CU3O7 AND FLUX PINNING

Growth spirals have been observed in the basal plane of bulk single crystals of Y-123 high T(c) cuprate, using scanning tunneling microscope with the unit cell resolution. The density of screw dislocations present is of the order of 10(7)-10(8)/cm2, close to the recently reported values for the sputtered epitaxial thin films of Y-123. The growth steps or terraces which are generally shown to possess a step height equal to the c parameter of the unit cell, have been further resolved into substeps corresponding to each of the triple perovskite block. Further, in the limit of ultimate resolution we have been able to demonstrate the substep heights corresponding to the individual atomic layers of the unit cell and thus provide a persuasive evidence that single crystals of Y-123 grow via layer-by-layer growth on the atomic scale within the purview of the spiral growth mechanism. Growth features are briefly discussed and it is argued that the observed screw dislocation density is still too low and the orientation is not favorable for realizing optimum flux pinning in these HTSC systems.