Nanostripe structures in SmBa₂Cu₃O_x superconductors

Atomic force microscopy and scanning tunnelling microscopy scans of SmBa₂Cu₃O_x (Sm123) high-T_c superconductors (single crystals and melt-textured samples) prepared using different growth techniques revealed the presence of nanoscale stripe-like structures, which are found to be sometimes parallel over several micrometres and sometimes wavy. These structures consist of chemical compositional fluctuations inherent to the light rare earth high-T_c superconductors and may act as effective pinning centres due to their periodicity of typically 10-60nm which is comparable to the ideal pinning centre size 2ξ (∼10nm for YBa₂Cu₃O_x in the ab-plane). Nanostripes are observed in Sm123 single crystals grown by the top-seed pulling technique and in melt-textured samples. The periodicity of the nanostripes is found to be much larger (∼50nm) in the former samples than in the melt-textured samples (10-25nm). Detailed measurements reveal that the nanostripes are formed by chains of individual nanoclusters formed from unit cells of the Sm-rich phase, Sm_1+xBa_2-xCu₃O_y. The control of these pinning structures running throughout the whole sample volume may be a key to improving critical current densities, especially at high external magnetic fields.