Direct observation of nanometer-scale pinning sites in (Nd _0.33Eu_0.20Gd_0.47)Ba₂Cu ₃O_7-δ single crystals

We report on the observation of self-organized stripe-like structures on the as-grown surface and in the bulk of (Nd,Eu,Gd)Ba₂Cu ₃O_y single crystals. The periodicity of the stripes on the surface lies between 500-800 nm. These are possibly the growth steps of the crystal. Transmission electron microscopy investigations revealed stripes of periodicity in the range of 20-40 nm in the bulk. From electron back scattered diffraction investigations, no crystallographic misorientation due to the nanostripes has been found. Scanning tunneling spectroscopic experiments revealed nonsuperconducting regions, running along twin directions, which presumably constitute strong pinning sites.