Stripe and criss-cross patterns in high-T_c superconductors revealed by atomic force microscopy and scanning tunnelling microscopy

By using atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) at ambient conditions, we have investigated the topographies of various surfaces of bulk samples of YBa₂Cu₃O_x (YBCO), SmBa₂Cu₃O_x- (SmBCO), and (Sm,Eu,Gd)Ba ₂Cu₃O_x (SEG) oxidic high-T_c superconductors. We find that the last two systems exhibit microstructures on the nanometer scale which are remarkably different from those obtained in the YBCO system. The stripe-like growth structures observed in our topography measurements may be the key for the considerable improvements concerning the critical current densities especially at high magnetic fields and elevated operating temperature (77 K). The properties of the microstructures are discussed in detail.